I think this game is great! Our 3 year old attempts to do the excercise and activities, but they are just beyond her abilities. I love the fact that fitness has become a "game".
Wii Fit has a fun set of games, with enough variety that you can choose what you want to do and avoid the games that drive you nuts! They system is well designed, with many of the exercises guiding the user to the correct form by the very nature of the design. It is challenging and many of the activities are a good workout.
I really love this game. I'm a total couch potato, and I've tried other home fitness equipment and gym memberships to no avail. I just can't stick with them long enough to do any good, but this has been a lot of fun. I've been playing three times a week for a month, and I can feel that I'm getting stronger.
On the down side I will say that my five-year-old daughter was unhappy to be rated overweight, so when she wasn't looking I adjusted her height a couple of inches, so now she's very excited to get a normal rating.
I'll also say that some of the fitness tips are less than helpful, and I wish it did more to help plan a workout strategy; for example, it already tracks which activities I complete each day, so it wouldn't be so much more complicated to come up with a recommendation of activities for the next day; for example, abs today, thighs tomorrow or strength today aerobic tomorrow. Something like that.
My three-year old son's favorite activity is running in place. He's hilarious. He pumps his little feet on the carpet like someone is chasing him!
For kids that don't get enough energy out by being inside on a rainy day, this is a great alternative and gets the entire family involved as well as helps establish goals.
My mother, my niece, and myself all play almost daily on my Wii Fit. For my niece, in particular, it was an amazing tool. We've been trying to help her lose some weight (on recommendation of her pediatrician), but nothing has really been able to help motivate her to do anything. However, after watching her Mii grow in size, she finally understood why we wanted to help her. And even better, she can play on Wii Fit for hours before we finally force her to take a break. She doesn't even really realize that she's exercising! It's perfect for what we needed.