I've used the Saris Bones (the three-bike version) for several years on several cars, none of which belong to me, so I have to be extra careful. It seems very gentle on the car, unless the plastic foot has been on the ground and has got covered with grit that scratches the paintwork. I've used it for long distances, with no problems -- Canada to North Carolina over two days. The bikes are held very tightly, and are very stable. They don't touch the car. I usually velcro the rear wheel so it doesn't move and send the pedals spinning around.

The only disadvantage I can see is that there's no easy way to lock the bikes to the rack, so I fret a little at freeway service stations. I tend to lock two bikes together with a cable lock -- it won't prevent the determined thief but makes it a little more difficult for an opportunist.