I use the wii fit for medical issues. I have weak muscles and poor stamina so it is good for me. It is very useful for strengthening muscles through the muscle workouts and yoga exercises and also for improving balance and core stablity. I have a daily regime of physio but using the wii fit as well makes it more fun as sometimes doing lots of 'bed exercises' gets boring! I also find the aerobic exercises good for increasing my stamina. But I dont use it instead of going out. So I would recommend it to people with similiar issues to me (as long as they are careful about wii-injuries!)
I also have a friend who has been using the wii fit to lose weight and she has been successful so far after only 2 weeks. She does not use it to replace going outside and walking or anything but she uses it for around half an hour per day to burn fat and tone. And people are already commenting!