Nah, don't want anything like that touching my car. Did try the magnetic ones back in the '80s...for maybe a few days before tossing them.
Consider that the paint would have to be *perfectly* clean before you attach them.
If it took a big hit, would the suction cups be moved around? Would they stay attached?
And you ever see how rubber suction cups can leave a permanent mark on windshields? I figure it's some kind of reaction between the suction cup and the "not-really-all-glass" windshield, and I'd sure hate for that to happen to my paint.
I think this is a cool idea for somebody to come up with to make money, but a poor idea to actually use on your car.