The American Council on Exercise recently conducted a study on the effectiveness of Nintendo's Wii Fit, and was "underwhelmed" by the results (PDF link). The study examined what the ACE dubbed to be the "most aerobically challenging activities," including the Free Run, Island Run, Super Hula Hoop, Free Step, Advanced Step and Rhythm Boxing. The study found that Free Run burned the most calories, averaging at 165 calories burned during a thirty-minute session. The next highest was Rhythm Boxing at 114, not exactly the fat-melting miracle one might hope for. Said Dr. John Porcari, "The Wii Fit is a very, very mild workout."
The ACE noted that performing the real-life activity -- real boxing, step aerobics, running, etc. -- burns significantly more calories than their Wii Fit counterparts. As an example, the study notes that traditional boxing burns three times as many calories as Rhythm Boxing. That said, the study did note that Wii Fit burns twice as many calories as normal video games, and that's got to count for something.
Still, if you're looking to really feel the burn, the study concluded that Wii Sports "is a better option" for gamers looking to find a decent workout.
Source - ACEFitness.org -- Wii Fit - Or Just a Wee Bit? (PDF)