Wii Fit has the possibility to make exercising fun, but it really won't make you lose weight.
In the end though the Wii Fit will not make you lose weight, not unless you put every ounce of effort into it and use it for hours each day. This would be fine but the actual exercises in the game just aren't captivating enough for that level of use. I found that repeating the same exercises day after day became a bit tiresome and there just wasn't enough to keep me interested.
The best way to use the Wii Fit successfully would be to not rely on it as a sole means of losing weight and getting fit, but more like an aid on the side. In this manner I think it is excellent and serves it purpose well.