My wife got me one for my last bday because I told her I would work out more if I have an xbox with kinnects. I did,,, till I threw out my back playing the star wars game.... the star wars game is fun, but its all 3rd person and not real responsive. Sometimes you swing your light saber, and the avatar just sits there.
I think that's how I threw my back out, getting frustrated and trying to make it work.
I tried a demo of a kungfu game that was also 3rd person, but they used pre calcualted sprites of you and it was totally responsive, but very 2d. Still it was fun, even in the demo version.
Blackwater is also a good one, but your virtual range is someone limited. Even the game that ships with the xbox kinnects was very good on controls. You really get a work out with them. I lost 10 lbs in my first week of playing.