I thought it an OK game. But first of all, I agree balance is important, but I believe the game is to centered on it (No pun intended). It doesn't really give you a good work out. I'm not denying that they aren't fun, I mean I could spend all day on them.
The only things that really can get you out of breath are the aerobics feature and I believe it should have been centered around that.
I found the running very irritating because it didn't give you much freedom to do anything but jog. The scenery and action was cute, but it didn't mask the fact the the running wasn't very good. I found myself getting on there just to watch the other Mii's jogging around. The features and unlock-ables were great, It's always fun to win something new. But they come to fast and easy.
The yoga was great. They really have a wide veriaty of helpful stretches. The trainers have a lot of good advice that really helped.

I was entirely fun for the first few weeks, but it got old fast.
You just have to be seriously committed to the game. Overall I found the game nicely laid out and fun for the whole family, you just have to be devoted.