I recently bought a Jawbone UP Move.
I'm not sure if they all work this way but here's how this one works: it simply counts your steps and acts as a glorified pedometer. The app, and not the device itself, is what makes it useful. The device has no idea if your steps came from walking to the store or doing sprints on a track - 1000 steps is 1000 steps. If you did do sprints (or any other activity) you then go back into the app and record your activity for a time period as well as your exertion level. The app then adjusts your caloric needs based on this new info.
The app also allows you to join a team with other people, and then you can see their activity throughout the day as well as their meals (you can hide meals, though) and activity. My GF and I compete to see who can take the most steps throughout the day. The Jawbone app syncs with MyFitnessPal app and imports my meals, which is terrific.
I don't think the info is accurate enough for you to become OCD about it. You could, I suppose, but it's really just giving you a wide picture of your daily activity.
The sleep function is interesting, not sure how accurate it is, though. It does make you more mindful of when you go to bed and wake up. My GF uses it while she sleeps, but I don't because I, uh...have no where to clip it to.