I've been debating buying a fitbit or a JawBone UP - I think I'm going to go with the fitbit now (I keep changing my mind).
The only thing I don't like about the fitbit is the design of the app - I've been playing around with redesigning it.
I'm still not sure of what would be the best visualisation of the data on a mobile device - so it's something I'll play around more with to see how I can improve it :)