I got mine last night! I was afraid it would be a little wasteful as I have a great pedometer (Omron), which keeps a log of all activity plus "aerobic" activity, which is basically when you move continuously for 10 minutes or more, and it's been extremely accurate.
But the Fitbit seems like a lot more fun. It's smaller so it's easier to have with me, and the clip makes it nice and secure. The sleep tracker is cool (the only good info I've synced so far). And I think all the graphs showing me minute-by-minute activity will be motivating.
I linked my Fitbit account with my Sparkpeople too.