My DS bought me a MiFit, the cheap Chinese version of the Fitbit, and I love it. He has one too and we're both a bit sad crazily competitive and a tad obsessive, so we compare our step counts. We've each got a target of 8,000 a day, and it does spur us on to be more active - I'll hop on my stepper in front of Corrie if I've had a quiet day. He says it's great for keeping track of his running stats, but I think my running days are over - the lower back pain put paid to it. What I really like is the way it tracks my sleep patterns. I was getting into the bad habit of staying up too late, but knowing the number of hours sleep, and deep sleep, I've had every night encourages me to compete against myself to do better.
I suppose it depends what sort of person you are - I'm afraid I need facts and figures to spur me on (as I said, a bit sad). I do regular yogalates sessions at home too to increase my flexibility (using the Louise Solomon dvds), and if I don't keep track of how often I do this and for how long I soon lose interest. Others will just get up and exercise and not need to know the fine details. Whatever works for you.