I'm using a vacuum mount (The Nukklehead, from Wood's Powr Grip) and have been impressed with the safety, not so much with the video quality. It tends to be very, very shaky under all driving conditions due to being mounted to the body (effectively, via the rear window in my case) and all the way in the back. I'm looking for a mount similar to what's being described here to try and get some better footage than what I've managed so far.
That said, I do plan on trying to get up the guts to mount it outside the car. I have a camera I wouldn't mind destroying if it came loose (my old one) but I'm afraid of the mount generation enough suction to warp the body panel I attach it to. And no way could I do that on a track, obviously. But a spirited run up a mountain road could get some freaking cool footage.