I did lose some fat and built more muscle while using Wii Fit. It has some good body weight exercise. However I also lost weight playing sports games on the Wii and I don't need Wii Fit to do body weight exercises. If you are going to use the Wii I recommend you include some fun sports games. You should also do some exercises on your own like jumping rope.
While I use the Wii as part of my workout routine I mostly use it as a form of active entertainment. I get most of my exercise while watching TV or while I am outside. Wii Fit has a lot of balance exercises. You use your abs and other core muscles to balance. It shows you how well your balance is and you can check your weight loss progress but checking your weight does not tell you how much fat you have.