I'm about five days in now, and I haven't done much at all of the "real" workouts on Wii Fit, and the games don't really have anywhere near the intensity that going outside for a long run does, but I have to say, it's sneaky. You start playing, and then after a few rounds of hula hooping or something you realize you're sweating.
And having the accountability of hoping on the balance board and it telling you if you;ve made progress is a frequent reminder to eat better and do a bit more exercising.
So yeah, I really like the thing.
And, heh, the kids like it, too.