Whether it's communicating easily with a buddy while riding, or listening to music rather than wind/exhaust noise for hours I can honestly say it changes the way you ride for the better.
Pros:
1. Good sound quality
2. crystal clear communication with phone calls and intercom.
(I've had full conversations with people, and they didn't even know I was on my bike going 60+mph)
3. Pairs quickly with your phone and allows you to start playing music.
4. Good battery life

Cons:
1. Could not get it to be recognized on my mac, I had to update the firmware on a pc
2. Latch that holds the Sena on the stand broke in under 2 weeks. (Currently awaiting my replacement unit)
I didn't even take it off the helmet, the clip just broke from holding it on normally. I'm hoping I simply received an older unit, and that
the problem has been addressed with newer production runs. The problem seems to come from the rubber seating where the device connects pushes the device up, while the cheap plastic clip tries to hold it down, eventually the clip fails and breaks from the constant pressure from the rubber seating.
3. The stupid part of the clamp that hangs down below the helmet constantly gets stuck on my leather jacket when I turn my head.
It's too obtrusive, and needs to be made much smaller.
4. Group intercom and pairing is sometimes a pain, specifically when trying to link different models, but when it finally works it's awesome.

So overall to make the unit perfect they would need to
A. Make the syncing of devices quicker and easier
B. Build quality needs ironed out.
(The part that broke on mine was a small plastic clip that holds the device to the helmet. For a premium device the latch should of been made of metal or a sturdier material. )
C. The bottom part of the clamp base needs to be cut down in size considerably. It hangs down way too far, and can make it dangerous when it gets caught on your jacket.
D. Fix the software for Macs so the device can be recognized (It is possible that my Sena 20s was bad, but I had no issues connecting to PC)

With all these issues, when the unit is working, it's glorious. I'm hooked on the ability to talk while riding, listen to music, listen to GPS directions etc. The convenience far outweighs the issues, and I can't ride without it now.
The company offers a two year warranty, and as long as they continue to stand behind their product, and fix some of these smaller issues that turn into major annoyances I can recommend picking one up.

(I'll update once I receive my replacement unit)

Update #1

I received my new unit. Sena honored their warranty without any hassle. They were prompt to help, and answer any questions I had. The only inconvenience was the wait. What I personally look for in any company is that they stand behind their product, and Sena has certainly done that. They now have a loyal customer, and I highly recommend picking one up if you were on the fence.

The new unit I received thus far has been working great. I did receive a newer production run unit, and the plastic clip that broke on my original has been replaced with a much more secure metal locking mechanism. It shows that the company does pay attention to its products. and constantly strives to improve them. That being said if your order a model, and the locking mechanism is plastic, pay close attention because it is likely to fail.

I waited a long time to purchase a comm system that I could use for bike to bike, GPS and music. There are a number of reviews on the Sena 20s but let me rate it as a 9 out of 10 on all it's functions. Saying that, the reason I purchased the Sena 20s is that it has all the pieces that I wanted in a helmet comm system. I wanted these functions in this order - music, GPS, and lastly bike to bike. In addition - as a big plus - I now have phone and text ability (for emergencies only). BTW, people can't tell I'm on my bike running at expressway speeds when I use it to talk on my phone. Another freebie this unit has is builtin FM radio. I also wanted a system that supported ear bud speakers instead of helmet speakers (typically use sPlug ear buds). I now use the helmet speakers while riding around town. They are the best that I have tried. My bike has a full radio system but I use mostly my cell phone for music. I use mostly web based music but also have music stored on the phone for those times where I have no cell service.
I've set up my bike where phone calls are displayed on my GPS. If I want to answer I answer with the Sena - not the GPS. I used to have my phone out in the open before I got the Sena. Now I use a jacket pocket or tail pack or fairing pocket to store it while under way. And no wires!

Pros:
Option rich
Great speakers and volume
Input jack
headphone jack
speaker/headphone options
good battery life
mic options - 3 mic options
easy function controls - volume, pause, stop, skip, pickup call, etc.
tap for command
Very fast phone pair
removable from helmet base

Cons:
sometimes have to repeat commands
has dropped signal for a few seconds (very rarely)
helmet speakers are good only at about 65 mph and slower (note - I have a quiet bike!)
double tap has to be done just right - this may be a plus in order to prevent false positives

Overall I am very satisfied with the unit and use it every time I ride.

tsp
In case you're wondering...
2016 R1200RT w audio and other stuff on it. Nav 5. iPhone (also works w Zumo 660)
Helmet: Shoei GT-Air
Also use w a BMW K1200S - nearly identical result but must always ride with face shield closed with helmet speakers.

So this was my first Sena device and my first time installing something like this onto my helmet. Running all of my wires was pretty easy but trying to connect the device to my helmet was a bit of a challenge. The regular clamp wouldnt fit around the bottom of my helmet because of the helmets design ( Voss 989 Helmet ) so i tried to use the double sided 3M tape mount and i cleaned my helmet where i wanted to mount it then used my heat gun on the tape to get it a bit melted and make it bond to the helmet better and that worked for about 3 days then when i got home after about a 20-30 minute ride i was taking my helmet off and noticed that the Sena attachment was barely hanging on so for that i had to return it

This is a full product review.

What You Receive:

In the box you will receive everything needed for installation. The main parts being the main 20S unit, high quality in-helmet speakers, microphones, mounting clamp/ hardware as well as USB cable/charger. Sena has thought ahead for installations in any type of helmet so they include 3 different microphones, adapters for mounting the unit, and some other small doodads that will be helpful in otherwise tricky installs.

Installation:

In my case I was installing it into a Shoei RF-1200 . The Shoei has sharp body lines and curves which made the standard clamp installation a bit more difficult versus a more standard rounded helmet. Thankfully there is sufficient space in between the liner and outer shell to get the clamp on the inside, however trying to tighten the nuts in such a small area proved to be pretty difficult.

The speakers were easier to install because the Shoei already comes with speaker holes cut out. All I had to do was properly place the speaker using the provided double side tape, and route the wires between the helmet channels.

If you spend a little extra time you can likely get a clean and strong install in almost any type of helmet. And just in case you can't, Sena provides a double sided adhesive mounting kit for those rarer circumstances. Sena also has Youtube videos of How To's for the Sena 20S, and I highly recommend watching those before attempting to install.

Functionality:

The 20S is quite impressive in its amount of functions, to achieve this they had to include not just 1, but 2 separate bluetooth chips working in conjunction. Some key points - 1.25 Mile Group intercom, Voice Control, Multi-bluetooth connections (e.g. Bluetooth GPS+Smartphone Bluetooth at same time), Audio multi-task (e.g. Smartphone GPS+Smartphone Music at the same time), Music Sharing, Sidetone, FM Radio, Speed Dial, Sensitivity controls, etc.

Admittedly with so many functions, it can be kind of a headache remembering what buttons to push to do X and spin dial to achieve X etc. Thankfully they have both computer and smartphone apps which allow easy to interface, full control functionality over most the Sena 20S settings. Also the battery life is very impressive, with a rated 13 hour talk time, and 10 day standby time.

Sound Quality:

Contrary to some reviews I've read, in my particular case with the Sena 20S paired to my Android device or my Mac, the sound coming out of the speakers is both very loud and very clear. Also phone calls via the boom mic are very clear as well. I'm not a huge audiophile but I have had my fair share of good audio equipment . On the overall spectrum of things, I would rate the sound quality as a medium to high for what it is. You're not going to get heavy bass out of these thin, small in-helmet speakers, but the highs are clear, mids do the job, and most importantly it can get loud without distortion. On the Max volume setting riding at higher speeds on the freeway, these are unbearably loud.

Build Quality:

Externally, it seems to be well constructed. The unit itself does not feel heavy. When fit into the mounting clamp it will make a tiny click to secure and I have no doubts that it will stay firmly in place even at high speeds. I haven't ridden with it in bad weather, but do note that this unit is not waterproof nor is it marketed as, it is only water-resistant. If you do get caught in a downpour you should immediately stop and take it off versus riding through and risking damage to the sensitive electronics. (Do you expose your cellphone to heavy rains?)

Issues:

This is what lost 1 star for me. My FM Radio function was only able to really pick up a few radio stations clearly. (I live in a suburban area of LA County) Many of the radio stations that I am able to tune into via my car radio or alarm clock radio and hear fine is just static/silent on the Sena. I contacted Sena support and after about a week got the response that I may have a faulty weak antenna in my unit. I purchased from an Amazon seller who is an authorized reseller, which means you get the Two-Year factory warranty on the device and I am currently in the process of an RMA with Sena.

I have also read other reviews about people receiving defective units be it DOA, FM reception or volume and this seems to point to a quality control issue that Sena didn't deal with prior to shipping these out. If you happen to be unlucky and do get an issue, you have the two year warranty to fall back on.

**Update** 8/26/14 This is a message I received from Sena Support

"After our careful investigation, we found the quality control of the earlier productions was not up to our standard. We took necessary actions to improve the quality control process and expect we will have a standard failure rates going forward. To make sure you do not receive another non-conforming unit as a replacement, we will send you a pre-tested replacement unit which is expected to be in stock in a couple of weeks. I will appreciate your patience on this matter. If you have any questions, please feel free to contact us again anytime. Thanks for your understanding and sorry for your inconvenience again."

**Update** 9/16/14

Received replacement unit. FM Radio works great! Changed rating from 4 to 5 stars.

Review Conclusion:

When you buy this unit, you're paying extra money for the new/improved functionalities that this bluetooth communicator provides. Which also means at times, being willing to rough through bleeding edge firmware updates and technicalities. You are not paying extra for a super premium product that is supposed to be the most user friendly or hardy.

If you just need bluetooth audio to listen to music or to communicate riding two-up, I would look for the cheaper Sena unit that can provide those functionalities with a smaller price tag. If your an adventure/touring rider that wants to ride through rain/dust (Though I would argue you really shouldn't have additional distractions motorcycling through hazardous weather), I would look to the marketed as waterproof/dustproof units that Cardo Scala Rider provides. (I have heard some models aren't fully waterproof either) If you need audio-multi tasking or a good multi-group intercom communicator, or are wanting to be an early adopter of new technology then you should shell out some more $$ for the 20S.

The main reason why I wanted to write a review is that I have found this unit has been getting a lot bad smack from people who are simply unaware of what they are spending their money on. The fact that there is that 2 year warranty with authorized resellers should quell most fears. Overall the Sena 20S is really good at what it is suppose to do and I would highly recommend it if it fits your needs.

It took a while to get it up an running. The documentation tells you to upgrade the firmware first thing, so that's what I tried to do. I use Mac computers and wasn't able to get any of my computer to recognize the Sena 20S, I tried 3 different computers. I tried to contact customer support for some help, but didn't hear anything. I googled for some help and found out that some say that the Sena Device software doesn't work with any of the last few recent Mac OS. So I booted to a pretty old version of Mac OS that I had on a spare drive. I was able to update the firmware with this work around. A friend who had the same issue, was able to do his update using a PC. I did finally hear back form Sena support, but after a couple exchanges they never verified if the Sena software works with some of the more recent versions of Mac OS.

I mounted the unit on a Nolan N104 modular helmet. The clamp attachment will not work with a modular helmet, the rim is too thick. I did find a video from Sena that show how to mount the unit on a Schuberth modular helmet, I followed those instructions and with a little adaptations was able to get everything mounted.

The 20S does a lot of things and because of this there are a lot of button combinations that are necessary to get the different features to work. I had no problems pairing my iPhone, Garmin GPS, and another 20S to use the intercom feature. A friend and I went out for a 4 hour ride and mostly used the intercom feature and it worked great.

The iPhone software for making adjustments to the settings, still thinks that I have the old firmware and so I rebooted my Mac to the old OS and re-downloaded the update, but the iPhone software still says I have the old version. Sena support has given me some instructions that is suppose
to fix this. Also, this iPhone Sena software doesn't seem to want to recognize a device when there is another device using the software on a different iPhone. Like, two people in the same room can't make adjustments.

I like how this works so far, though it was a bit of a challenge to get it mounted and up and working. I look forward to using some of the other features