It's not going to be a saving grace for your health, but it could help.
Wii Fit uses the balance board to do four different things. Yoga, strength training, aerobics, and balance. You're not going to find anything out of the ordinary in yoga. Strength training is obviously to tone your muscles. Aerobics is meant to get your heart rate up and burn calories. And balance... well I don't know how balance is going to help you in the long run. The game seems to have a love affair with posture and is the first thing it talks to you about and then keeps talking to you about.
If you do a lot in Wii Fit every day, you will definitely work up a sweat. Yoga is a lot harder than it seems, and you'll probably find yourself swearing at your trainer when he tells you that you're balance is a bit off (thanks to your leg being jammed behind your back and your arms going to places you never thought possible.) Strength training is more doable, except push ups. Unless its the first thing you do, you're probably going to have sweaty palms, and you're probably not going to be able to keep your hands on the balance board.
A good thing Wii Fit is for is keeping track of your weight and body mass index. At the very start of the game it takes your weight and bmi and lets you set a goal to try and achieve over another set amount of time. This visual aid will definitely help you try and achieve your goals due to its ease of tracking.
Over all, Wii Fit isn't a game, and it isn't a god of health. But like I said, I'm sure if you combine this daily with other exercise, you will definitely get more in shape than you were before.