Hi Billy,
I equipped my boat with 2 of their products. I have their 4 cup holder with the 4" suction cup and I also mounted a fan on a 4" cup to defrost the windshield. Their main advantage is that they don't have to be permanently mounted and leave the surfaces for other uses as well.
They are on the expensive side, but really hold well. Joe's right. They must be mounted on a smooth surface.
Kent
MARCIA JANE