I wear one...
I have a Misfit Shine I got (subsidized) through my work.
I wear it on my wrist.
It also acts as a watch.
I'm out of shape and I know it, and like most of you I sit at a desk all day with occasional forays to the vending machine.
I have to say that I'm walking more because of it. Instead of sitting all lunch catching up on the latest TV show I missed I go for a walk instead. The goal is 10,000 steps per day. If I get half of those in before I head home I'm going to hit that target, so I AM walking more. Which is good.
Since the start of May I've dropped 7-8lbs, and while I could have done that on my own, the Shine, I think, has been a motivation. I've been wearing it for a month now, and it's not going anywhere but back on my wrist.