I quite enjoyed it when I played around on a mates, but there are issues; bmi isn't a reliable way of checking health/weight, some of its yoga and strength based workouts are pretty poor (and aren't great for demonstrating form), the running events don't deal with the risks associated with high impact training and it doesn't really deal with the idea of an effective warm-up.
As an exercise tool studies have shown it's only really effective for those who live a sedentiary life outside of it; it makes no real difference if you already have an averagely active lifestyle.
That said... it's still fun and every little helps. Don't expect it to really do anything for you longterm, but it's a good start and could well be a solid place to build up your base fitness level. Still if the choice is between doing a half hour Wii Fit session and a half hour of "real" exercise, the "real" exercise will be better for you... but Wii Fit isn't bad.
It's good for what it is; a fun game that hopefully gets some people exercising who otherwise wouldn't and does some things quite well (I've heard great reports on its aerobic type exercises), but don't see it as a replacement for a traditional exercise regime.
I'm interested to see what they do with it next though: surely downloadable exercises (possibly even user generated) should be the next step.