I have a wii fit and if you put the effort in, you can work up a sweat! Lol. It can be a bit boring as Jaylou says, but it really is more for the whole family...my 3 year old loves it!
One thing I would say though is if you do get it, do not go by the scales that are with the programme as these are definitely not accurate!
Its ok, but there are others on the market that are just as good...would I get it again??? Probably not for the reason it was intended, but as fun...yes.