I use it every night.
The app actually wakes you averagely always 5-20minutes before you set the timer. (you set it to wake you up at 7.30am and it goes off at 7.23am)
The app has to run and the iPhone has to lie on your bed, so the movement sensor can "feel" whenever you're moving/turning in your sleep during sleep. This is what allows the app to know the sleep phases you're in, because in same stages you don't move in sleep!
Basically the device wakes you up in one of your movement stages, so that it doesn't have to ****** you out of a dream or deep sleep stage(don't know the exact english words).
The technic mentioned above is also the reason why the app needs a lot of battery, which is no problem if you connect it to the charger while sleeping.
And don't forget the app also records all sounds.
Pretty nice feature: you can listen to music which stops playing after a set amount of time before sleeping. So you may listen to relaxing music which automatically turns off after X minutes.