I've had my Band since launch; I preordered it. It died last Thursday. I basically experienced the same thing people are describing here. A couple of times I've found my Band dead, not having noticed any low battery warning. But each time I was able to charge the Band with no issue. This time, however, I was sure I had another day's charge left and it refused to stay on.
So I took it to the Microsoft store in my area and got a replacement. I didn't get any explanation and the employees didn't spend any time trying to troubleshoot the thing. I noticed that they're now charging $175 for it, which is depressing because it implies they're not selling.
I guess a positive is that I got a second cable out of this. If this replacement ends up giving me trouble and I get a third, I'll probably end up selling that one on eBay.
Microsoft has some serious quality control and/or design flaw issues here. All too often, it seems like it's not a matter of it, but when their products will experience problems.