The Band is the perfect example of Microsoft's ability to come up with a great product but completely botch its execution.
I owned the first gen for a month before returning it. The thing actually died within that first month due to the ingress of sweat, and that was without doing any strenuous activity. I didn't want to be stuck with something this prone to failure, but the actual functionality was fantastic. It worked well, and it had GPS.
So when the Band 2 was released I decided to get that, assuming and hoping that issues had been sorted out. The device is definitely much improved, but there have been two major issues. One is the band tearing and the second is some kind of battery failure. I experienced the second issue 6 months in and got a replacement. As others have mentioned, getting the replacement was effortless, but these kinds of problems shouldn't be happening at all.
The big frustration for me, however, was that smartwatch functionality never worked properly between my Lumia 950 and the Band. You'd figure that Microsoft's own devices would work with each better than with anything else. I've since gotten an iPhone, but continue to use the Band, and it works reliably like it never has with the Windows phone.
It really is a joke how Microsoft is incapable of getting their shit together to make reliable products. It's also disappointing that so many opportunities have been squandered. The flaws aside, the Band is a fantastic device.