I got a Fitbit Flex. I like that it has an API so other developers can integrate with it, a nice dashboard, and a nice mobile app. It also integrates with a health channel application we're using at work, meaning a little less duplicate data entry.
The big thing with this is the daily goals - it helps a lot to track when I'm getting steps in and how much water I'm taking in (which is important since I'm trying to prevent kidney stones).
To track my weight, I got the Fitbit Aria so I could have most of my data in one place.