I just bought an aXbo and last night was my first trial of it. I'm lovin' it.
But I didn't buy it to wake during REMs. From my understanding it actually is meant to wake you after REM. It monitors your movements, so it won't wake you up when you're in the no-movement time. I bought it especially because I'm tired of being woken up in the middle of dreams by alarm clocks and hoping this one wouldn't.
Last night I set the alarm for 8 o'clock maximum and it woke me up at 7:43. I had just ended a dream and was somewhere on the limb between sleep and vigil.
Supposedly it gets better with time, after the clock learns my sleep cycle, but for first trial I was positively surprised, because it worked perfectly although my sleeping cycle is nothing short of unusual...