It's fun and a pretty good incentive to move about. That said, we were pretty bored with the included songs and to have access to all the songs you have to get a monthly subscription which was a bit too much for something that's just fun.
My wife is a big fan, drawn in by the songs she recognizes and the promise of exercise. But ever since we visited Japan and tried the rhythm games in the arcades there we've both felt Just Dance to be a bit lacking -- the tracking isn't very sensitive and has a huge tolerance for accepting wrong/mistimed moves