I also agree with Hoser on the Wii Fit--it is more of a light workout. When I first started using it, it helped because I was, as tmhiggins said, a couch potato. However, when I hit my plateau I think it was because I had become active enough that it was no longer challenging me. I then switched to Wii Biggest Loser and am loving it! When I do the workout, I always notice a difference afterwards, and, if you put an effort into it, will work up a good sweat.
Believing you can is half the battle!