I was skeptical at first, but now a month later I have to say I love my Fitbit!
It tracks my steps, floors climbed, sleep patterns, calories burned. If you put in just a few minutes of time entering what you eat/drink throughout the day it takes care of all your calorie counts. And when you first set up your account you enter your age, height, weight, gender and goals so it takes care of the math in figuring out how many calories you should eat each day.
You can manually enter a variety of activities in addition to what the fitbit tracks.
I'm finding it very motivating, at a glance I can see where I stand that day. If I'm craving a cookie but I have almost used up my calories I find it easier to say no (or run around until I've bumped up my activity enough to justify more calories).
My only complaint? That the user can't adjust the day. At midnight a new day is automatically started, my "normal" day has me going to bed around 4am. Not a big deal, but it would be more user friendly for people that work non-traditional hours if that could be changed by the user.
Best of all? With the help of my fitbit keeping me on track and honest, I've lost 10lbs in the last month! :D