I bought my Band less than a month ago. I've been wearing it daily, but primarily during work hours. I've been really happy with it.
Unfortunately, this weekend it started acting up. First incident I was wearing it and discovered it was dead. I assumed I had forgotten to charge it and did so. But then later that day and yesterday I found the thing completely off. Both times it came back on, but since then it's shut off and refused to come back on. Today I had it sitting on my desk between meetings and when I came back I was able to turn it back on, but after about 20 minutes on my wrist shut off again.
What I'm experiencing is exactly what I'd get from a device that's gotten wet. There must be some short somewhere inside this thing, mostly likely where the batteries reside. I haven't done anything more than going for walks with the Band, but I do sweat while wearing it. There's a small gap between the battery covers and Band so I suspect that's where sweat and humidity get in.
This is massively disappointing because I really do love the thing. I haven't had a problem with comfort or anything. That said, I'm going to return it. It's not even a month old and I'm already having problems. I don't want to bother with a replacement just to end up with another unit that will eventually fail as well.
At this point I'm going to wait and see how the new Band turns out at the end of the year. As good as the current model might be, I do feel now like Microsoft rushed to market. Some of these issues seem fairly glaring and obvious and probably would have been addressed had they given themselves more time for refinement.