I have the FitBit Alta. I like this model because it's simple, small, and easy to charge (unlike the flex which has to be taken apart to charge).
Some people here think fitbits are a gimmick but in my case, I really like doing the challenges. This is where you make friends with other fitbit people (my family and co-workers) and then there are a variety of competitions. I have never walked so much as I am constantly trying to beat my co-workers. Just last night, I walked every isle of WalMart twice because I was close to winning the day. Yep, I won
