According to https://secure.wikimedia.org/wikipedia/en/wiki/Fitbit uses a 3D accelerometer, so it seems to be more complex than just a pedometer. I've found it to be quite accurate for daytime activity, especially tracking when I'm being really active. But I'm just now starting to use it to measure sleep (and to compare the results to the WakeMate while I'm at it), so can't speak much to its effectiveness yet.