Web MD reviews the wii fit, and concludes:
If you're going to substitute giving your thumbs a workout with the Wii video games for a Wii exercise game, that's a good choice, in terms of actual activity and caloric expenditure," he says. "The hope [however] is that if we can engage people in trying some of these sports through Wii, we can entice them to try the real thing and get some real exercise.
According to the article linked above, many of the activities do not burn as many calories as doing the actual activity. I've been trying to decide if wii fit is a good way to stay in shape, or just a fun side activity.
The four areas are; balance, aerobics, strength training, and yoga.
The "balance exercises" are games, and I don't see any physical benefit to doing them, I haven't decided if doing them regularly will improve my balance or not.
The aerobics exercises start out very easy, but I think I'm in fairly decent shape to start with, the hula hoop has gotten more difficult as I've done it more, so perhaps the others would too.
The strength training has some exercises that I find challenging and some that I don't think do anything, since there are no weights to make them more difficult.
The yoga is probably my favorite. I've never done a lot of "real" yoga before, so maybe those who have wouldn't like it as much, I'm not sure, but I like how the balance board can tell you if you are holding the position the way that you should be.
My conclusion is that it should not replace all other physical activity. It won't replace a gym membership- it can not replicate weight training especially, which I think is critical for women especially to maintain bone density (can you tell I'm pretending to know what I'm talking about here?). It is fun though, and if that gets you off the couch (and in my case, away from the computer), it's a good thing.
There is a place on wii fit to enter in outside activities into your log, although short of having all your workouts logged in one place, I don't particularly see a benefit.
I'm still sore from hula hooping, and the jackknife challenge, so if muscle soreness is any indication of a good workout, It was one.