So, I got the Wii fit the day it came out and I've been using it ever since. It is great! It tracks your weight and BMI over time (it is a scale), and it walks you through various exercises, some easier than others. You can also set a weight goal and timing for it, and it will track your progress towards that goal.
The Wii fit offers yoga exercises, aerobics, strength training, and balance. The best part about it is that it shows you where your center of balance is during each exercise, so it forces you to have really good form and posture -- making the exercises more difficult, but more rewarding! The other thing that is noteworthy about the wii fit, is that it comes with built-in trainers who demonstrate all the exercises for you (and do them with you). They tend to go slowly, so you end up getting a lot more out of each rep than you would at the gym, where you might tend to rush through your reps. Anyway, that's a high level overview. Let me know if you want me to come back and blog more about the fit or other fun exercise stuff to do while dieting!