I used a super cheap one in college and currently have a Bell 2-bike model now. No issues with either of them, especially with a single bike on them. The key, as stated above, really is getting the straps good and tight - tighten them again after the bike is on it; and having a clean surface on the car and clean pads on the rack. Even a thin layer of dust between the pads and the car will cause scratches.

I rarely carry more than one bike on mine. If I have multiple bikes, they go in the back of the truck, or on a hitch rack.

CL is often great source for racks.