That doesn't mean that stuff like this isn't useful for normal people who don't flail around like an animal when they sleep, but an anecdote is an anecdote. I'd also imagine a more sophisticated piece of software might be able to learn your sleep pattern and adjust the sensitivity (i.e. calibrate the threshold based on how much you personally move when you sleep, rather than the hypothetical average person).
(in Wakemate's defense, they have an awesome no-questions-asked 30-day return policy, so I was happily able to return it and get my money back when I wasn't satisfied. They were a true pleasure to deal with.)
So worst case is that you wake up the same way you would with a normal alarm clock, but on the other days its pretty neat.
But the 30% battery charge I lose overnight is sometimes a hassle because my phone is either near the 30% right before I want to go to bed or I know that I have no time to charge it in the morning... and I refuse to charge the phone while its laying next to me :)
Example: http://www.sleepcycle.com/
I've used this but only for a few times because it requires the phone to be activated all night, which makes it quite warm in the morning. But it seemed to make me less groggy...but I can't tell if that's just confirmation bias...
I haven't thought of using it for just 45 min spans though