A lot of good and right answers.
Longston, I use those cheap rubberqueen ones now, I have the flap tucked under the center console on both sides. It works ok for 3 dollars a peice. When I have guests in the car that I dont want them to see it, I can remove it without leaving a scar or mark.
The ones sold at the dealer are expensive, poor design, and can leave marks on the doors. You can use the adhesive instead of screws but it proably wont be as sturdy. If not placed properly, those dealer ones get in the way of your leg/knee and can risk spilling stuff. Also, if you have it attached to the door and a soda can in it, when you slam the door shut when getting in or out of the car, suprise! you might have a little soda to clean up.