I bought these for my wife, since she likes to run. The problem I had is it was missing pieces of the accessory kit. I returned it and requested a replacement and it was also missing pieces from the accessory kit. This seemed to be an error from the factory, as both had the seal. There must have been a batch of these (or maybe I got lucky with the only two that wasn't packed correctly).

PROS
1. EAR FIT. I put this first because this is the biggest problem with all Bluetooth headsets/buds (IMO). Without a good fit, the sound isn't good and the device is annoying. This is the 4th BT I've owned. 3 of them were in the $100 price range. This is by far the best fit. I'm using the default fins. It has 2 extra fins, 2 silicone buds, and 2 foam buds. Lots of choices for the correct fit.
2. SOUND. Great sound. I'm not an audiophile but I do listen to a wide range of music from classical while reading to EDM while working out. Anything in this price point will have good sound unless the ear fit is bad.
3. APP. The app is amazing. It has directions for everything from the technical setup to the fit of the ear pieces with illustrations. The app has a customizable EQ that is amazing by it itself. It even has 100's of community preset EQ settings. You can switch the L and R speakers in the app (first time I've seen this). You can pair with multiple devices at the same time; like your phone and your computer so you can still get a call while listening to music (again this is a first for me).
4. CHARGING. At first the charging adapter was a little off putting. But when I realized I don't have to take the time to find the micro USB hole to plug it in, it's became a definite plus. Attaching to the charging adapter is simple and faster than a micro USB.
6. NOISE CANCELLATION. I gave it 4 stars for noise cancellation. It doesn't have noise cancellation like a Bose headset. However any BT device that plugs into your ears does dampen sound. With no music on, it really dampens the noise. With music on, it's hard to hear anything else even when the music is low. My test was using it in a restaurant at lunch time. I have not used it on a plane which would be the real test.

CONS
1. BUTTONS. Minor and picky. The button symbols are recessed instead of raised. However after I use it several times, I suspect it won't matter because I'll know which button regardless.
2. CHARGING ADAPTER. Minor and picky. The charging adapter is 3 inches long. You will likely need to get a USB extension unless you have a plug that isn't near the floor. They only cost $3-$5 for a 3 foot extender.

NOTEWORTHY
BATTERY. Some would consider the 8 hour battery life a huge con. However this unit is tiny. The fact that it puts out 8 hours of uptime for such a small unit is a plus. I would almost expect 6 hours out of something this small. There are other BT devices that have 10 or 20 hours. They are also bigger and or more expensive. This is designed to be a small workout device and not a 20 hour plane ride to Australia device. (Who works out for 20 hours?)

Overall I recommend the X4. At the time of this writing I paid $86. I think this is excellent for this price point. I feel like I got the value and more.

I mainly use these as running headphones. I run outdoors, mostly in a quiet-ish city neighborhood. Occasionally will use them as everyday walking to class headphones if it's raining.

Previous headphones: For a long time I used the apple headphones that came with the phone and a bunch of other cheap wired buds. Before this, I used Powerbeats Wireless 3, but over time the ear hooks snapped off (not an issue as they stayed just as well) and the remote stopped working (major issue as I'd try to pause/lower the volume and could only make them louder). I tried the Elite 65t buds before this.

Sound: I will admit I'm not an audiophile of any sort, but they sound great to me. I use one of the preset EQ settings, just messed around with them until it sounded good to my ears. Noise canceling is fine. I run outdoors, so I actually want ambient noise and I haven't been run over yet so good by my mark. For the average bear, I'd say the sound quality is great.

Comfort: Not the most comfortable earbuds I've ever worn, but good for running. I've worn them for 2+ hours at a time, and it's fine. Def, not an all-day set though. Personally, I hate a tight seal for running buds (I don't like the "full feeling" and need ambient noise to not die), and found these are great for me. Especially as compared to the 65t Elite which created too solid a seal for me.

Features: I'm a chronic music switcher so having an inline easy to use remote with pause/forward/volume was super important. The remote is super easy to use while exercising, the buttons are a good size and have a nice clickability. The long press fast forward/reverse took a little adjustment but it works really well and doesn't mess with my flow. The fact that they're waterproof is amazing! I hate running on a treadmill, and so will go out in all weather. These have survived a few runs in pouring rain along with a lot of sweat and they're going strong.

I bought my second set as a used one, so that was a great price but it didn't come with all the different pieces like a new set. It only had the (very) large rubber tips and the ear hooks were large on one side and small on the other. Good thing I still had the foam tips and hooks from my broken old set.
I really like the sound quality and noise isolation I get with the foam tips, obviously not as good as active noise cancelation but good enough for daily use. They're really light with just the small controller on the cord, so I can run with them without anything flopping and banging around my neck. I like the corded ones, so I can just pull them out of the ear and leave them hanging around the neck without finding a place for the buds each time.

My use cases for sports earbuds:

- Resistance training in the gym, streaming from stationary iPhone
- Running on treadmill in the gym, ditto
- Running outdoors, streaming from Apple Watch (series 3)
- 99% of the time I listen to podcasts; not music
- I sweat. A lot. Like, seriously a lot.
- Not a long-distance runner; 4-6 miles tops

I've never had a good experience with earbuds and cardio exercise. Between the sweat and the vibration, they have universally been more annoying and distracting than helpful"”halfway falling out, actually falling out, getting swamped with sweat, etc. I've been running without "entertainment" for a couple of decades now and it's great, but sometimes a guy wants a podcast. So finally decided to try some sports earbuds. After a couple of weeks with these, and after some work and experimentation, I'm hesitantly pleased.

The experimentation is mostly about figuring out which of the several fitting options works for your head and ears. It's a bit finicky, and takes a few minutes when putting them on, but every time I run with them I think I'm getting closer to a configuration that actually feels secure and allows me to just listen without reaching up to push them back into place every 10 strides. Amazing! Life-changing! Ok, maybe not life-changing.

I had a few outdoor runs where I thought I'd have to return them because the Bluetooth connection to my Apple Watch was unlistenably bad. What a piece of crap! I thought. But I thought I'd try one last thing before giving up: switch the side of my head the receiver lies against so that it is on the same side as my Apple Watch. That did the trick"”now the connection's fine. So: pro-tip. Unless it's only my head that's dense enough to block bluetooth (quite possible).

(I have since noticed the fitting instructions suggest putting the receiver on the left, which is my watch arm. Guess I should read instructions more carefully. ^_^ In any case, props to Jaybird for allowing you to switch L/R in software.)

Other notes. Waterproofing working so far"”at least my prodigious amount of sweat has not had any noticeable effect . Sound quality is more than adequate for podcasts, which again, is mostly what I listen to when exercising. Pretty decent for music too, but they're obviously not studio-quality cans. Battery's fine for my use cases so far. A recharge every few runs isn't a problem for me. We'll see how well it holds a charge over time and use, of course. Connecting to/from/among different devices"”iPhone and watch for me"”is fiddly and error-prone, but that seems kind of par for the course for bluetooth devices.

Overall, I think it's a little too fiddly in the fitting and connecting department for five stars, and maybe some warning about the bluetooth receiver sensitivity would have been nice. But after all these years, it's great to be able to bring a podcast along on a run!