Here's another question:
How many Wii Fits are out there getting used 3-7 days a week by 1-8 people?
I bought a Wii Fit. I use it 5-7 days a week. One roommate uses it 5-7 days a week. Another uses it once a week and another uses it once a month.
Also, most people are bums who give up on any exercise routine ever, so this is actually a pretty good number for Wii Fit. I see more unused treadmills and bikes and rowing machines than unused balance boards.