I too have it and really enjoy it. There are some who would say that it isn't much of a workout but I doubt they are doing the strength exercises if they are saying that. Besides strength all the aerobics workouts are great as well. The yoga is also effective, but I don't really do it much. It's like a much funner version of a personal trainer added with video games. And you get to create a cute little mii character who will cheer you on I'm sure.
I've been doing it (strength mixed w/ aerobics circuit style) for about 45 mins a day and I am really noticing a difference in how my clothes fit. Only been doing it for about a week though, Still, very very fun.