OMFG!
It's so fun. DS got it for Christmas and begged me to have a "dance-off" with him. Previously I've thought I'd look a tit, and I was right!
But still, after half an hour I've sweat like a sweaty thing and out of breath and feel pretty great. That half hour flew by, too, because I got caught up in gaining stars for perfect dance moves. 
I have about 2 stone to shift and hate exercise (except swimming) so if anyone else is like me and is looking for a fun, unconventional way to burn some calories, you should give this a go!