I have a Thule receiver hitch mount and have no problems with it at all. It has been on 3 vehicles (Subaru, '07 T4R and even my '06 Honda Accord) for over 5 years. Not once has a bike (road and mountain) been damaged or come loose (and I've not babied it).
I like the the hitch mount because I don't have to take off the wheels (and risk forgetting to properly attach them and set the brake). I also don't have to lift the bike onto the top of the car/truck after a long day of riding. And, if I had a garage, I would not have to worry about damaging the bike or the garage because I forgot that it was on top of the vehicle (this usually happens to those people who have a bike that is 3x as valuable as the car it is sitting on top of).