I've had my Fitbit one for almost 2 years now, and I'm fairly addicted.
Pros: I love how small and indiscreet it is, and that it's easy to slip into a pocket or clip on clothes. It seems that the number of steps and the elevation changes are fairly accurate. I like the emailed summary of my activity each week, and the email notification when the battery is low. Long battery life, too.
Cons: While it's great for counting steps, it does not account for other activity, so if you do strength training, you don't get "credit." That doesn't bother me though because, to me, the steps are bonuses on top of regular workouts. The size of the device makes it prone to accidental washing or being lost. (Hasn't happened to me yet.) And my first silicone case wore out so I had to replace it, but the tracker itself is great.