I downloaded this app to try it out, the one I downloaded is called sleep cycle. I hate waking up in the mornings, and I work whenever I want, so trying to stick to a routing of waking up early doesn't really matter for me, and I think my body knows this. I don't know if I'm a good judge of this app, but I don't feel any different than I do when waking up with a regular alarm set. The accelerometer tracks your body movements as you sleep, no movements mean you're in deep sleep, many movements mean you are in a light sleep. I can see how it tracks how I've been sleeping with the statistics it keeps. Maybe it's just me, but I feel no different and wouldn't waste the money on it.
-Slevin