I just bought a Wii Fit last week. Kinda cool. I don't expect any real exercise, I'm just trying to get the whole family off the couch/Computer/laptop. Seems to be working.

As for any of the martial arts, I took Wado Ru Karate for over 12 years and after the first year it stopped being good for a primary source of exercise. It's great to do, but without the extra cross training it becomes less and less of any type of aerobic benefit. BJJ or MT may be more so than some of the other arts.