The step counting feature is accurate as far as I can tell. My daily goal is set at 10k and most days I've been able to reach it (it doesn't hurt that I'm usually beyond 3,000 by 7am thanks to Insanity). My highest day was 15, 624! During the day, I wear the one on my waistband and only once have I changed my clothes and forgot to transfer over the lil guy.
At night, however, the One comes with a wrist band to house the FitBit while you sleep. Don't ask me how, but it tracks your sleep - how long it takes for you to fall asleep, times awakened, times in bed, and sleep efficiency. I don't get enough sleep - plain and simple. But this tracking has shown me that at least of the time I am in bed, my sleep efficiency is in the high 90% range, waking up on average about 3 times per night for no more than 3 minutes.
Another thing that I love so far is the battery life - I've had it since last Tuesday and haven't charged it once. The battery is said to last between 5-7 days but I'm going on nine and trust me, I play with it constantly. However, the questions remains: When I will charge it when the time comes? because I don't want to miss out on steps OR my sleep tracking. Perhaps while I'm in the shower. If anyone has one and can give me some advice - I'll take it.
I wish there were a heart rate monitor associate with this device so I could get more accurate measures of how many calories I was actually burning during workouts, that would be rad - but for the time being, this is suiting my needs just swimmingly.
If you're looking for an activity tracker, I give this one (so far) my glowing stamp of approval. And if you already have a FitBit, we should connect - you can compare steps with your friends. (Sorry Gorman, I'm still smoking you!)