I've done a few workouts on the Wii Fit. It is a fantastic and fascinating workout instrument. However, there are some significant limitations you should be aware of.
1. The BMI ratings and Wii Age are completely off for children. It told Andrew that he was actually 13 years old and overweight (he's a perfectly healthy little boy). Do not put any stock whatsoever into what it tells your children.
2. You can not easily workout with any partner. The game starts with you choosing your Mii and everything you do is recorded under that Mii. This also means that player 1 must completely finish their workout before player 2 can begin. You cannot do multiplayer except for one 2 player running aerobics exercise (and the second players stats are not recorded).
3. For each 1 to 3 minute exercise you do, you need to keep the Wiimote handy because you have to hit A several times for each exercise. After each exercise, you have to go back to the main screen and select a new exercise. It is extremely annoying to not be able to program your workout so that they go from one to the next automatically (or at least with hitting the "A" button once).
4. Number 3 above also leads to spending an excessive amount of time just navigating the console in Wii Fit. You will not be able to complete a 30 minute exercise in 30 minutes, it will be more like twice that time due to all of the starting and stopping in between each exercise.
It's a fascinating first attempt at interactive workouts on gaming consoles, but it definitely has plenty of room for growth.