Got one for Christmas after using the iPhone app for a month on its own. I had so much fun pushing myself to the next threshold for a badge, that my wife decided to get one too. This kind of device is great for exactly that- showing yourself how active you are (or aren't) and then getting yourself to do just a little more. At the moment I'm covered in sweat from racing my wife to the 100 floors climbed badge.

As far as features, I love the One for its ability to tell how many floors I've climbed. It adds a little variety that is better than just a step counter. The website and iOS app allows you to fill in all the kinds of activity that the One can't measure. Watching sleep activity is neat. Turns out I'm a heavy sleeper and my wife wakes up pretty frequently in the night. The calorie counting aspects are pretty in-depth, although I don't use it. Interactivity between FitBit and other websites like EarnedIt, IMapMyFitness and Fitocracy are cool too. I'd prefer the wristband version if it had a display and altimeter like the one does. Maybe next year.

I'm hoping they continue to clean up and update the website and app. There also seems to be a lot of demand for an iPad specific app. 9/10.