Ive been thinking of getting a couple myself. People pass many many parked up vans everyday and don't take much notice most of the time. Stick an A board each side and you instantly draw more attention to yourself.
For what they cost and the minimal space they take up on the van i think its a winner, it can't do any harm.
I love the 0800 repair vans with the big screwdriver on the roof, not sure how us carpet cleaners can do something similar though