If I hadn't paid an extra $50 (for an already expensive watch) for the "music" capability, I would be much more satisfied. The bluetooth on this watch is junk. I have to switch the wrist my watch is on every time I run or the music cuts out constantly. Not sure if they can fix this with a software update in the future or not, but I really hope so.

The Forerunner 645 Music has performed well for as what Garmin promises, straps are easy to change, Battery lasts one week while using it with the heart sensor 3 runs during the week of one hour and listening to music. I was a previous Fenix 3 user and the music and additional Garmin Pay is a great addition to the Forerunner Series.
Not everything can be perfect, I have only used the Airpods to listen to music and they have performed well even though they are not on the list of Bluetooth headsets of Garmin. While using the RD (Running Dynamics) Pod while connected to the Airpords the music disconnects, I hope a fix for that comes soon or an explanation to it.

UPDATED 4/11/18: I have not yet contacted Garmin as they responded, but I plan to do so. However, I did notice this past Sunday (when home most of day) that my watch battery lasted much longer than normal after I recharged it. It led me to think that when I am away from home it may be searching for a new WiFi network. I don't want to add my work WiFi, which can be weak. I can't figure out if there is a way to just "Turn Off" WiFi on my watch, so I just went to my Garmin app and removed my home WiFi network. For me personally, I don't think I really need the WiFi, since I sync to to the app on my phone. After 3.5 days (including one-Music only run and one GPS-only run for about an hour total) my battery is still at 50%. This is a huge improvement! It makes me feel much better about the watch. Now I'll just respond to Garmin and hope some of the other kinks can be worked out, because I really do like the fit and most of the features.

Original Review: I was very excited to get this watch. As a female with a smaller wrist, I was excited for the smaller size. I had still been using my trusty old Garmin 205 on runs until just about a year ago, and tried running with phone apps over the last year. I was excited that this watch would get be back to running with fewer cords and gadgets.

Starting with the pros, the watch does fit very comfortably and looks nice. It fit me much better than the 935 and the 235 I have tried on. For me personally, I tried it on a run with GPS and music yesterday and didn't have the connectivity issues some other people have noted with their Bluetooth headphones. For reference, I was using SoundPEATS which aren't highest sound quality but connectivity was decent on run (wearing watch on my left wrist).

For Cons, I have had two issues with the watch. First, with the battery life. It is advertised as up to 7 days in smart watch mode. With the colder than normal winter and icy conditions, I haven't been able to get outside as much as I would like to. Therefore, strength training and treadmill running have been my go-to exercises. And since I had to research and order headphones, the first week and a half of use was without music. Even with no GPS tracking and no music, I can't get the battery to last past 4 days. (Which concerns me because the battery lives only seem to deteriorate over time.)

I got my headphones two days ago, and a 35 minute outing with GPS and music used about 17% of my battery. I thought this was a fair, maybe slightly below expectations, based on the advertisement of 5 hours with GPS and music. The next day (on same battery charge) used it on a 4.5 mile walk for about an hour with GPS (no music as i was talking with a friend). When I was finished, my battery was still above 50%. However, less than an hour later, my watch buzzed and told me I only had 10% battery left (this is only about 36 hours since my last complete charge). I noticed the watch wasn't connect to my phone either (which was in range.) I have read a few other reviews that noted issues reconnecting to phone after GPS use I am not sure whether my watch was killing the battery trying to pair with the phone or what it was doing (it was not in GPS mode, i checked), but something zapped the battery.

The second con I would like to point out is something I have read in other reviews. On my test run with GPS + Music, my watch reset itself at exactly mile two, after the voice prompts of my distance and pace. (For reference, my software was updated to the most recent version of 2.5 two days ago.) I would have been very frustrated if I were out for a hard run, as it made me stop wait for green triangle screen to clear, restart music and make sure my run was still being tracked.

I really want to love the watch, it fits and looks nice. But I am seriously considering whether it is worth the extra investment for me personally.

I want to start by saying I've owned four Garmin Forerunners over the years for my running and they all worked great from the beginning. Returned this 645M after just a couple runs. I bought this primarily for music so I wouldn't have to have my iPod on me (yes I still listen to music locally - no streaming). On my first run, the headphones dropped connectivity. I had to fumble around while running to try an reconnect (not safe to do outside near a road). Apparently, wearing the watch on your left hand when the Bluetooth controls for the headphones are usually on your right side can cause connectivity issues. So they recommend wearing the watch on your right hand for certain headphones. My headphones (Jaybird X2) were "certified" to work with the watch on either wrist. Not so well I would say. So, when I tried to do a system reset it just froze at the big green triangle. Finally had to hold the power button down forever to get it to turn off. Seems several people are having these types of issues (check out the Garmin forums for this specific model). They even refer to it as the "Triangle of Death". That combined with the five hour battery life if using GPS and music just couldn't justify the price. I mean with that battery life there can be no margin for error if I want to finish a marathon without the battery running out. So I'll wait a while and see if they improve it or wait until the next product refresh. My Forerunner 235 I bought last year still works like a champ. I just wanted music built in to avoid carrying my mp3 player with me.

EDIT: I want to say that I could live with the battery life for most of my runs. I knew about the battery life before ordering. But that combined with the connectivity problems and device freezing right off the bat when doing a reset didn't give me any confidence in the product. So back it went.

Bought this as a replacement for the Forerunner 225. This review is largely based on that perspective.

Pros:
- Smaller, looks like a normal watch on my skinny runner's wrist.
- Smart watch capabilities.
- Heart rate monitoring is actually useful, always on and seems to measure accurately while running.
- Add on "widgets".
- Stress monitoring is an interesting idea and works well.
- Customizable watch faces.
- Battery life seems to be better than I expected.

Cons:
- Wrist band picks up dirt/dust/sweat stains pretty easily.
- Can't seem to turn off semi-random notifications (example: performance score) during a run.
- Can't clear missed call notification (restart phone is all that seems to work).
- Default watch face doesn't seem to allow real time heart rate display.

In general, this is a very nice watch. A big upgrade from the 225, as one would expect when moving from a pre-smartwatch 200 series to a 600 series smartwatch. To be honest, I had to nitpick a bit to come up with the cons.

GPS/run functionality while on the run works in a very familiar way, while building on top a lot more tracking functionality.

The extra tracking is probably not a game changer for those of us who have learned to listen to our bodies but is interesting for after the fact tracking and to put a number to what we were feeling during the run. Kind of an after the fact verification that we're not crazy and maybe a way to quantify those things.

Smartwatch features are very nice. As a tech geek, I especially like the extra apps that can be installed. Pretty cool to be able to do things like control my smart home from my watch.

The all day stress tracking and heart rate monitoring are nice. I'm a bit surprised with what the stress tracking could pick up on. I came down with a cold just after getting the watch and could track the course of the cold with the stress tracking. Not really telling me anything I didn't already know by listening to my body but, as with the tracking on the run, a way to see after the fact that I'm not crazy and, to some extent, quantify how significant the stress was.

With its smaller size, I was concerned about battery life, especially given the 24/7 heart rate monitoring. As it turns out, it appears battery life is as good as, if not better than, the 225 even though I had the heart rate monitoring always off on the 225. As of now, I charged almost 4 full days ago and have had GPS on for over 3 hours. I'm still at 50% battery.

Note: I did not get the "music" version as I don't listen to music while running and have my phone with me at other times.