I have one and love it. There are a few other threads around here about it. I'll look around and see if I can find them.
It is basically a more advanced pedometer. The sleep function is probably helpful if you are worried about apnea but won't tell you how well you're sleeping if you have insomnia but stay in bed quietly (me). I like having challenges and I track things like how many flights of stairs I go up and what percentage of my day is sedentary (way too much!). If I had the money I would go for the bodybugg or whatever they are calling it now, but the FitBit doesn't require a subscription so is more affordable for me. It's definitely motivated me again and I needed that.
Found a few, there are more threads comparing the various devices:
http://www.3fatchicks.com/forum/does...ar-device.html
http://www.3fatchicks.com/forum/does...ed-fitbit.html
http://www.3fatchicks.com/forum/does...35-fitbit.html