I have a Nike+ FuelBand... not that impressed really. It looks good and tracks steps just fine but it's far from feature rich like the Fitbit and no-way close to the upcoming Amiigo. Very underwhelmed.
fitbit one and the aria scale. so far so good. setup is reasonably quick and easy, website and iphone app are pretty clean and intuitive, but I've only used it for about a week. also, I haven't tried the others, so I don't have a good comparison.
The bigger question becomes what changes does one make after they buy one. What do you do with the data? Does one take meaningful actions when they see how sedentary they are or how many calories they have eaten (if they log food).