I have had my Fitbit One for two weeks now and I continue to be satisfied with my purchase. When I first got it, I opted to have a 'lazy day' to see the number of steps on what used to be a typical day and I found that I only had 1628 steps. Talk about lazy and embarrassing!
Fortunately, seeing this number was a bit of a reality check. The predetermined daily goal is 10 000. That's a lot of steps. You can change this goal if you would like. For myself, I'm satisfied with 7000 steps on a day that I've done an intense workout. I opted not to change the number officially though because I figured that would make it too easy to slack off.
In these two weeks, I have averaged 8198 steps per day. That is a significant improvement over the 1628.
Other general features that I enjoy include the silent/vibrating alarm and the ability to link my Fitbit to My Fitness Pal. And the fact that it can track your sleep is pretty cool, although not that accurate or useful.
In terms of the actual device, I opted for the One over the Flex (which is a bracelet) because I wanted to be able to quickly see my numbers without having to check my phone or computer. (The Flex just has lights that show your progress). I do still use the website so I can get added details and see how I'm doing over time.
I can understand the appeal of the Flex. It would be much harder to forget to put it on and you are less likely to throw it into the wash, but these aren't issues that I have encountered with the One. You can compare some of the features here.
Anyway, I realize that my review isn't super in depth or earth shattering, but I did promise you a review - so that's what you get.