For what it's worth, I'm returning mine... Here's my two cents:
1. GPS is broken. Badly. Never had any issues with the original Band. It would take it a minute or two to lock onto a GPS signal at a new location, but after that it would always acquire a signal in matter of seconds and stick to it. I'd say the device functioned on par with any of Garmin watches (I have both Vivoactive and Fenix 3) and was much-much better than Fitbit Charge or Adidas SmartRun. The accuracy of distance, pace, etc. was also spot on. When I first read reviews about the new Band saying GPS appeared slow I laughed. How can it be? Band 1 had it absolutely right, sure Band 2 would be an improvement? Unfortunately, no. Got the device on October 30 and went for a run the next morning. The reviews were right: it took it good 4-5 minutes to find the signal, but find it did, and the accuracy seemed comparable to Fenix 3, so I was content to let it be. Then over the next two weeks GPS functionality got progressively worse. It took it longer and longer to lock on the signal, it started dropping it until yesterday when GPS would no longer work at all. I tried everything: rebooting the phone and the Band, resetting the Band, using different phones and OS (went from Windows Mobile to Android). GPS is dead. After just two weeks and a handful of runs.
2. Heart rate is meh. The original Band blew my mind. I know it's personal, but I tried all sorts of optical heart rate sensors under the sun, and they all sucked for circuit training, free weights, and kettlebells. Basis B1 was laughable. Fitbit Charge was a bad joke. Wellograph I don't even want to remember. SmartRun was anything but smart, and Scosche Rhythm didn't quite dance. Even Apple Watch that does a very competent job with running, walking, and rowing couldn't handle it. Band shined for me. Yes, it would lose heart rate here and there, but it would recover real quick. Just stand still for a second or two, and it locks the rate again. Band 2 certainly didn't improve things at all. If anything, it now loses track of heartbeat quickly and then starts chaotically jumping around trying to "guess" it for the next 10-15 seconds. By the time it finally matches the strap monitor, your exercise is long over, and your heart rate is 15-20 beats lower than it was. It also tracks worse when running.
But it's rowing that really gets me! Basically, as soon as I start rowing the heart rate jumps to 150 (always 150 for some reason) and stays there no matter what for as long as I row. Doesn't matter the real heart rate. Doesn't matter the intensity. It's 150 baby, and it will stay that way for 2, 5, 10 or whatever minutes until you stop rowing, at which point it promptly jumps to your real heart rate. Start again -- and you will be back to 150 within half a minute.
3. Battery Life is Atrocious. One of the reasons I dropped the original Band for a while was its battery life. You had to charge the damn thing every one or two days which compared to 10-12 days with Garmin Vivoactive was pretty bad. Microsoft advertised Band 2 would have solid 2 days of battery life. Really, Microsoft? Doing what? Playing dead? Because for my usual day (10,000-12,000 steps), which would include 30-40 minute run with a GPS on and about 30 minutes circuit training, the battery is below 15% by the time it's evening. I have to take it off my arm and charge it every day, religiously. Or there will be no Band the next morning.
4. Sleep tracking is useless. I know I'm biased after Basis amazing sleep analysis, but I tried other trackers too: Up, Charge, Shine, Wellograph, Vivoactive, Vivosmart. Band 1 was hardly any good showing I was awake in the middle of the night when I clearly wasn't. Band 2 didn't improve situation at all. According to it, I spend hours at night wide awake (and have no recollection of it afterwards! Neither does my wife though). My "restful sleep" that was paltry 1.5 hours according to Band 1 went down to 20 minutes if at all. If we were to believe Band 2, I'm a stressed out insomniac with little or no restful sleep on a verge of a manic attack.
Now, it may be that my Band 2 is simply defective. Dead GPS and quick discharge time are certainly signs of some hardware trouble. But as everyone else, I was burnt by a clearly rushed and still rough Surface Book that is still flaky after firmware and 10586 updates, and we all know the truly pathetic state Windows Mobile 10 is. Don't even get me started on OneDrive egg-on-face. So, if you are considering buying Band 2, the question you should ask yourself is this: do you want to give $270 or so to a company that is clearly no longer interested in making quality products? And if you can't believe the data coming out of your tracker, what's the point of having it in the first place?