I had the Fitbit One for about three days, it seemed like a good product. I was gathering a sufficient amount of data to get a sense of its accuracy and consistency, it appeared to be quite successful at doing its job. Unfortunately for me, it slipped off my belt and is now lost forever. For what it's worth, I think the functionality of this product is decent and I recommend trying it but only if one finds a secure way to fix it to one's body... shame to have lost $100 though, probably won't invest in it again.
gotta love a device that claims i climbed a flight of stairs when i did no such thing and then fails to record a climb that i did do. useful? i don't think so.
then there's its supporting website and iphone app: the website displays weight data incorrectly. enter 71.7kg and it displays 71.6. not consistently but often enough that it's obviously programmer error and not user. also the iphone app displays that particular data correctly. where the iphone app fails at is if you enter 750ml for water consumed, it displays 25 fl oz.
the overall impression is of a clueless company that only halfway developed their products.