Wii Fit only sold well because of the hype behind it. I own a wii, and a few of my friends do as well. A couple of my friends bought Fit and used it for maybe a week before that balance board got put into the closet.
It doesn't really do anything, but everyone was so hyped up because it was supposed to be a "cool" way to exercise with video games (not that this idea hasn't already been around forever).
I just hope if Sony does this, they do it right and give people real workouts while making it fun to play.