I carry a bike on the back of my car a *lot* and it's just far, far simpler to use a hitch rack. I have had all kinds of bike racks on all kinds of cars and trucks. Nothing, IMHO, beats the latest generation of hitch racks .. the ones with the wheel trays like the Thule T-2 and Yakima HoldUp.
Advantages:
o The wheels stay on the bike
o The car is aero
o No slamming bikes into the garage, etc.
o No mud on roof
o No worries about putting a bike on the roof
o No worries about mud and debris falling from bike on to roof
o ...
Disadvantages:
o Hitch receiver
It would be really, really hard for me to use a roof rack as my primary bike rack. That's how awesome these wheel-tray hitch racks are for regular use.
The roof racks do look sporty though!