From experience, the effect is that your alarm goes off almost like you were expecting it. It's as if you are "sleeping 'til you wake up" but somehow an alarm is involved. I love it. Hopefully, gone are the days of hearing a strange beat from the far side of my dream, slowly and confusingly realizing I was actually sleeping and not knowing what is going on before I remember that Disco Inferno is my alarm and I'm supposed to be waking up. (Yeah, yeah, it's a run-on, but I think it really lets you experience the confusion I'm talking about.)
Apparently your body moves, or doesn't as the case may be, depending on which cycle you are in. This app uses the sensitive accelerometer in the iPhone and monitors your movements, resulting in a graphed record of your sleep. I'm a graph person. I love analyzing and finding patterns. To me, the alarm feature is just an added bonus.
At left is my graph from last night. At 2am, you can see I came out of a pretty deep sleep since my dog had put me in such an uncomfortable position. I kicked her off. Then at 6:30-ish am, my 2 year-old wanted to get up. I coaxed her into going back to sleep. (You can see that shortly after 7am I was in the deep, deep throes of la-la land, so I'm not sure when she crawled out of my bed.)
The cool thing is that after a few nights, I've noticed a pattern. One, I've been entirely horrible about going to bed at a reasonable hour, but that's nothing new. Second, I've noticed that I am consistently in light sleep or even awake in the six o'clock hour. If I can just go to bed earlier, it seems as though I should be able to wake up "for good" at that early hour. Heck, I just might become a morning person after all.
PS - This app costs $.99 and is well worth it in my opinion.