I know this is an old post, but it was at the bottom of our email today, so I read it and wanted to respond. I have the Wii Fit, Wii Fit Plus, Your Shape, EA Active and Golds Gym Cardio. I have tried all but the Golds Gym Cardio (which I won).
I have poorly formed knees and EA Active is a killer on my knees, so I stopped using it. Wii Fit Plus is a fun addition to the Wii Fit, but neither work up a great sweat. So far I like Your Shape (also got for free), but it's a little quirky with the camera (you have to make sure it is calibrated correctly and the background is free of distractions.
Have you heard about kettlebells? I just learned about them after reading a Tweet that talked about this study: http://www.acefitness.org/getfit/research.aspx (right below the kettlebell blurb is a link to an article about Wii Fit).
Supposedly you can burn up to 1200 calories an hour with the kettlebells. I think you must have to really work out with them to get that kind of burn rate, but I am starting off slowly with a few DVDs and library books and a set of kettlebells and I definitely feel the workout. While it's not something you can do with the Wii, I thought I'd mention kettlebells as an alternative if you are looking for something you can do at home. Good luck!