What I really like about Wii Fit, is that the user experience does a great job of making excercise, something that you normally don't want to do, fun and actually enjoyable. One unique thing that I like about it is the "Wii Fit Channel" which allows you to do your daily fitness test without having to put the game disc in the system.
This might seem like a minor feature, but it is actually a very nice addition and makes it easier to stay aware of your weight and BMI. It is also a good example of a feature that directly supports a user's goals. The goal might be to "check my weight everyday". Sounds simple enough. Why would they go the extra mile to allow you to do that without putting the disk in? The about a real user in real life. Chances are users are adults. There is a good chance that those users also have kids. Imagine a real situtation where kids are always playing different games and you have to pry the controller away from them. The time it takes to take the old game out, find the case to put it in, find the Wii fit disk and put that in probably adds up to more time that the test takes. All in all the process of just weighing yourself would seem much more tedious if you needed the disk.
So, combine this with all the time of have be spending playing Rockband, and I might just get back into good enough shape to get my old college band (I'm the guitar player) back together.
Its really interesting to see how video games these days are really changing the way we think about them. When I got my first Atari 2600 back in 1981, video games were something that you played while sitting on the couch...though... my thumb really got a workout on that single red button.