I used it for a month or so. It does work to an extent, but isn't 100% reliable. It also won't work if there are two people in the bed or on a memory foam mattress.
Basically you leave your phone on the corner of your bed, and the acceleromater measures the movements you make in your sleep. If the alarm is set for 6am, if you start moving/tossing (I guess coming out of REM?) around 5am it will go ahead and wake you up rather than let you fall into another REM cycle.