I was one of those early, pre-release adopters of the Fitbit Flex, and because of that luck, I've been able to enjoy my Fitbit for over a month now. It's been enough time that I feel comfy about a review.
First off: I had no idea how much a slug I am as a remote tele-worker. If I'm not paying attention, I can walk less than 2000 steps in a day. That glaring realization in itself is enough to justify the Fitbit. :-D
I chose the Flex because I wanted a device that never had to come off. No clipping, no pocketing, no remembering. As a bracelet, it's always on. It was a no-brainer.
The setup was a piece of cake, and the Fitbit Dashboard is a really neat way to see your stats and the status of your goals on a daily, weekly, monthly basis.
My goal is 10,000 steps a day. In a month, I think I've been able to hit that goal about 5 times, so it's not as easy as you think. I mostly average about 8000 steps. The motivation hits when I realize I'm only at 5000 steps, and I get off the couch to get up to 8000. Soon I will be averaging 10,000 --- but it's hard.
One of the neater features is that the Flex can tell if you are lightly active, moderately active or vigorously active. I don't know how it does it - but it can tell the difference between me walking around the house, and me walking the dog vs cutting the grass with a push mower. I think it's magic. :-D
The sleep monitor is neat, but I'm not sure how accurate it is. That being said, it *does* give you a good indication of whether you are a restless sleeper, or a nice, deep sleeper... :-D
I don't use the calorie tracker/food input options, and I probably should. I'm just not good at tracking food intake, and I'm not sure that's important for me.
I wish the Flex tracked stairs, but it looks like the Fitbit One is the only device that does that right now. I also wish it had a GPS, since I love seeing my walks on a map.
Overall --- it IS motivational. I am looking forward to more friends getting the Fitbit, so we can compete against each other --- and I think that's where the real value is going to come in. Fitness mixed with social media is going to be the motivator for me!