Anyone who does proper exercise could have pointed this out, but it's always nice to have it in writing; the American Council on Exercise have claimed that Nintendo's Wii Fit produces "underwhelming results".
The group has released a report on the super-popular home fitness program, drawing on research performed by the University of Wisconsin. And this report has found that even Wii Fit's most physically taxing workouts can't hold a candle to actual exercise.
Wii Fit's boxing, for example, burns only one-third of the calories of actual boxing, while the other four most intensive modes — Free Island Run, Super Hula Hoop, Advanced Step and Free Step — only burned between 100 and 160 calories for every 30 minutes of exercise. Considering a cheeseburger has around 300 calories, you'll be on Wii Fit all day if you want to really burn some fat.
Perhaps most damning/hilarious, however, is the report's finding that while Wii Fit burns more calories than a regular game — where you're doing nothing — it's not as good for you as a session on Nintendo's own Wii Sports.
Ah, the power of marketing. American Council on Exercise Charts 'Underwhelming' Wii Fit Health Benefits [Gamasutra]