I promised myself I wasn't going to buy one, then spontaneously did so anyway. That's what being a woman is all about.
Anyway, I guess my overwhelming impression thus far is that it's significantly better than I'd expected, but my expectations were admittedly rather dismal. I also picked up Your Shape: Fitness Evolved and Dance Central, both of which are pretty good. I bought Fitness Evolved to supplement my current exercise routine, which until now had comprised daily cycling and sit-ups and the occasional swim, and the cardio/toning stuff in the game (uh, "game") has significantly improved my overall workout. There's so much content stuffed in there, I know I'll be spinning that disc for months yet.
I bought Dance Central more for the novelty of it than anything else, since the music is really not at all to my tastes, but it's great fun with a crowd. I had a bunch of friends over for New Year's Eve, and Dance Central (and alcohol) pretty much keep everybody entertained the entire night.
It's a pretty cool bit of tech - I suppose I'm a slut for gadgets, which is probably why I relented and bought the thing - so I'm hoping to see some devs do something really interesting with it this year. In particular, I'd really like to see games featuring a combination of Kinect support and additional peripherals.