I'm using this application for a few weeks now. The application needs to be calibrated for the first few days, but after that it works really well. I'm pleasantly surprised by the accuracy of the application and the ease of use. The low price ($0.99) makes it easy to try it out too, so if you care about these kind of things, I can recommend it.

One thing I've noticed is that in your sleep, you're sometimes rolling over while being "half awake": you wouldn't remember it if you would fall back asleep, but if you're inside the specific timeframe you've set your alarm to, this application will wake you up, and you indeed do feel more fresh than coming out of a deep sleep.

And the charts are nice too.