OK, so as a lover of anything promoting exercise, I'm all for the Wii. Using it as your only form of exercise, that I'm not so sure about.
If you are looking to maximize your Wii workout you should check out these Wii Weights. Talk about feeling the burn while bowling, these Wii weights wrap around your wrists to make those backhand swings really count on the tennis court.
Although these weights are just a prototype, adding weights to your Wii time is a great idea, especially if the game is your only form of exercise. If you're really serious about it, you can go out and get this weighted vest or some wrist or ankle weights. You'll be building muscle in no time.