I've got one and when I first got it and used it all the time I used to come off it shattered but had lots of fun, my and my husband would be fighting over whose turn it was!
I think I made the mistake of then trying to do all the yoga everyday and 30 mins jogging on the freerun section a day which made it boring and I never had anytime for any of the fun stuff so I got bored of it and have stopped using it (I'm ashamed) lol
What kind of difference it makes to weight loss I'm not sure..... the 'fun' games don't burn many calories but I used to happily spend hours doing them.
I suppose what I would say is if you're a keen been exerciser anyway then it's unecessary but fun, so might be worth investing in if you can afford if you're more of a couch potatoe and can't be bothered the fact that it's games and can compete with yourself or others might give you the motivation to actually bother :0)