Is it stable enough for longer trips (over 500 miles)? Yes

Does the rack itself leave any marks or indentations on the car? No

Does it hold the bike far enough away from the car that they don't bang into it? Yes

Is there anything else a prospective buyer should know? Certain frame sizes and shapes may not fit on the rack: Tiny (<50cm) frames can potentially be a problem, non-cyclindrical and/or steeply slanting top-tubes can potentially be a problem, if it's an S&S-coupled frame the rear brake cable coupler can interfere with the rack...

but it really is a very nice rack nonetheless.