We've got a couple of Thule carriers that fit to the roof of our Audi A3. They're really well made and look like they'd stand up to some abuse when left on the roof at a service station (not that anyone wants to nick a bike with no springy forks, right?). I leave them on for most of the summer. They're really easy to use once installed on the bars. I remove the whole assembly, bars and carriers, each time I don't need it.
Bad points are that I can't seem to move the carriers forward on the roof bars. This is probably me being daft, but the position of the bars is definitely fixed. This means a little spoiler at the top of the back window now has a dent where it collides with the carrier when you open the boot. It would be OK on a longer car. Ours is a fake estate. Also, as garrema says, fuel economy nosedives if you go anywhere above seventy. The bikes get covered in squashed insects too. I tell people they got there 'cause I ride so fast. no one believes me.
T..