I have an Arkon DM320 with the adapter plate and vent mount adapter. I can remove the vent adapter from the vent in about 30 second. The trick is to pay attention to how it "snaps" into place. So far I never needed to swear at it while taking it off, that should show you how easy it is.

With my setup, I have a Sebring Convertible, the Axim screen points a little low, but nothing I couldn't get used to. The nicest aspect of a vent mount, is that you do not have anything blocking your view, and the mount will not fall off.