I've had one for a while now. I really like it. I don't usually bother to track my sleep as getting up in the night tends to mess it up a bit for me. The steps, distance, floors etc are usually very accurate, but stormy weather can mess up the floors, and it's worth remembering that you have to do a whole floor (10' I think) for it to count, just running up and down 5 steps won't count.
It can be linked with MFP, so you can track your food on MFP, and the fitbit will automatically increase your calorie allowance if you've been more active than usual. Or indeed the other way, if you've sat on your arse all day it will deduct calories! You can add work outs manually, such as doing the 30 day shred for 20 odd minutes, as the fitbit won't record the full calorie burn by itself.
I was doing the "eat more to weigh less" thing for a while (which is great!) and I found the fitbit invaluable for that. My work can either be very stationary, or I can spend most of the day on my feet, so it's great to see how active I've been, then if need be I can hop off the bus a few stops early to bump up my steps. Or I can smuggly ride all the way home! 
The forums are really good if you have any problems with it too.