I have them on my truck and really like them. First of all, I like the look of the truck better with them on, but more importantly I don't have AC in my truck so the windows are pretty much always down and it helps to have the shades to keep the rain, etc. out. I've also noticed that they help to block the sun through the window as well.
They attach with black metal clips (4-5 per door) that seat under the window rubber along the top of the door. It's a super easy installation and you can take them off and put them back on at will without damaging the truck. Here's a for sale post that shows the clips:
http://67-72chevytrucks.com/vboard/s...ght=vent+clips
Just also fyi in case you didn't know, Ventshades are an aftermarket product that are still being sold new by Lund International, so you can buy them new pretty much anywhere. You can also just get new clips if you need them directly from the manufacturer.
Hope that helps.