Like I always say, if you find some way to exercise that you enjoy, well then, enjoy! But since I'm all about keeping fitness low tech and low cost and easy to do just about anywhere, I have to wonder about the Wii Fit. It kind of spendy, doesn't give you the same workout as the actual activity it simulates and it keeps you tied to your TV indoors.
Here's a wrap up on a study done by the American Council on Exercise which basically says that any exercise is better than none, but none of the Wii activities burn as many calories as the actual sport. I also found this parody of the Wii Fit.
If you have a Wii Fit and you find it keeps you off the couch, by all means go for it! But if you're looking for a way to get in shape maybe the actual activity--dancing, hula hooping, bowling, tennis, etc--might be more enjoyable in real time, with real people. Personally if I were tied to a computer all day the last thing I think I'd want to do is watch another flat screen at home. That's why I take it outside!