I got a Wii Fit this summer. I was working out on it for like 45 minutes a day while the eldest played outside and the youngest took a nap.
I was making improvement. My weight wasn't dropping off like mad, but I was getting rid of some and I was toning. I felt better. Also, an advantage for me - I HATE working out and as this is a game, it was never really a "workout" in my mind. (I play these little "head games" with myself!)
I think you can use it as a fitness tool. It's a good fitness thing for me because I hate going to the gym, and I honestly don't like to work out. I was working mostly on aerobic and balance. I LOVE the boxing! My arms hurt after starting that, but then I totally got into it and did that lots! And because the Wii Fit tracks your progress, I could see improvement in my balance. My weight was going down, but not by leaps and bounds. And then I got pregnant so I quit. But as #3 is due April 16, I plan to start using the Wii Fit again as soon as I'm able. And, I can't wait!