Wii vs the actual sport

Using a standard measure called a metabolic equivalent of a task (MET), researchers have rated the effort required in the Wii video games. 

Overall Wii sessions aren't as tough as the activities they mimic - for example real tennis takes 7 METs but the Wii game only 3 to 4.  Although you're more active playing or doing the real thing, Wii does help you meet the American Heart Association's recommendations for optimal fitness: a half hour of moderately intense activity ( 3 to 6 METs) 5 days a week.

Nine Wii activities- including basic step class, baseball, and tennis - demanded 3 to 4 METs, putting them on par with brisk walking (3.8 METs).  Wii push-ups, advanced step and boxing called for more than four METs. 

Information from 3/10 Readers' Digest