Sure they work, they make tons of money for the people who sell them. Oh, you mean work as in will it make your abs look like the TV commercial? Duh, no.
There is some theory behind them, physical therapists use some kind of similar device to help with certain kinds of muscular therapy. But believing the TV commercial is like saying, Bill Gates uses computers, therefore if I use computers I'll be as rich as Bill Gates. Leaves out the important part.