I've actually had the chance to play with one of these firsthand, and I must say, I'm quite impressed. The only basis for comparison I have is the wii.

Initially, there is some confusion as to what it is you're supposed to do but after a minute of awkward movement you get the hang of it. This feels much more natural than the wii, and the lack of controller is freeing. The full body movement detection is fun and really encourages you to move around. On the other hand, the sensitivity is not that high. It's okay with the casual games it comes with, but I'd like to see some more examples of, say, shooters.