I have tried it and like it a lot. It is fun and allows people to experiment with many different modalities of exercise. I am particularly impressed with the visual feedback for the balance exercises. I have seen people sticking with the Wii Fit exercise program because it is interactive and engaging.
At this point in time, I am willing to endorse just about anything that will get people off the couch and moving even if it does not initially conform to guidelines for ideal amounts of exercise. And the Wii Fit is as safe as any exercise video out there.