We bought one for Christmas and have basically been playing it non-stop. It's amazing. And, we've been completely satisfied playing just the game that comes with the Kinect (which is an adventures-type game where you can do lots of different things, like white water rafting, dodge ball-type thing, flying in space, etc).
We also have a Wii. The Kinect is, IMHO, much better. The technology is just better, which makes for a cooler gaming experience.
I'm not what you'd consider a "hardcore" gamer. In the past year, I've played the Kinect, Deadrising 2, and on the Wii. That's it. So, take my feedback for what it's worth (which isn't much, probably! ).
I have an old XBox 360 - like I bought it when it first came out. I have done nothing to it (i.e., I haven't done any of the modifications or whatever that you can do). To play the Kinect, I needed two additional things that I didn't have: (1) a flash drive for more memory and (2) to connect to XBox Live so that I could download an update, which took like 15 seconds to download. That was it.
Hope this was helpful. Let me know if it wasn't.