Paul Krugman explains:
[W]hat fitness devices do, at least for me, is make it harder to lie to myself. And that’s crucial. It’s all too easy to convince yourself that you’ve done enough walking, that shuffling around filing books is a pretty good workout, that you only miss exercise once a week or so — OK, maybe twice. But there’s your Fitbit telling you that you only walked 6000 steps and burned 1800 calories yesterday, that you only did serious exercise three days last week.
You might say that the truth will show up on the scale and your waistline eventually; yes, but that’s too future oriented. You need to guilt-trip yourself in the here and now.
Yes. And since I've started counting steps every day, and making decisions that result in even more steps, I've lost 7½ kilos—one stone two, to my UK friends—and brought my resting heart rate down to 60-65. I've also been able to correlate sleep quality with mental performance and diet, which doesn't mean I always sleep well or long enough, but it at least helps me plan my days better.
Oh, and I bought the Fitbit Surge, which is even cooler than the Flex I've been using.