Everyone I know is wearing an activity tracker. At first to be honest I didn't get it who cares how many steps I take or how long I sleep or my continuous heart rate. From what I have seen and found out is these little devices are a window into your overall health. The even help a lot of people to be more active and burn more calories. I have only had mine for a few days so I'll have to let you know if it really helps or if it's just a novelty. I chose the Fitbit HR in Plum which just came out because it has the best ratings on the web, here's a review:

Pros: Smooth and slender look. Includes caller ID and vibrates with incoming calls. Takes your time and heart rate with the ability to choose practically any activity you may do. The continuous heart rate tracking pushes it above most of its competitors because this allows for a lot more analysis.

Cons: No GPS means tracking won’t be as accurate and no pace or distance tracking for the avid runner. Offers extremely simple sleep tracking. Battery life is not so great. It’s 1ATM water resistant so you can wear it in the shower but that’s pretty much it.