I used the Body Bug for two years and loved it. Note that it is worn around the upper arm using an elastic band so that is a negative. What I liked about it and have not found in any other wrist devises is that it doesn't just calculate movement or forward movement. Thus, it tracks all types of workouts such as stairmaster, lifting weights, etc. It also tracks the intensity of your movement so it can calculate calories burned on say, a high intensity elliptical workout versus an easy one. Also, it knows the difference in effort and calories between doing sprints versus running at an easy jog pace. Not sure how it does this, but I think it was some sort of electrical impulses, skin temp, movement, and pulse (although it does not give heart rate info.) You buy the devise and then pay a monthly subscription fee to use their software, enter food intake, etc. Can be used with an iphone app. It does not monitor sleep, except for the fact that if you move around a lot in your sleep it will show more calories burned during the night.
To me, the advantages were that it tracked every type of activity plus the intensity. The disadvantages were monthly dues and wearing on my arm (not very professional while at work if wearing short sleeves.)
If I could find a Fitbit type devise that tracked everthing my Body Bug did, I'd buy it immediately.