I regularly carry my £4K Santacruz and my partners £2.5K Orange on the roof of my car using Thule 591 carriers. I've done thousands and thousands of miles with them up there, at fast motorway speeds, in gales, rain, snow, everything and I've never, ever had an issue (touches some wood).
I just put them up there and drive, they really are very secure in my experience. I care very much about my bikes and wouldn't use the roof carriers if I didn't think they were 100% secure. The bikes stay pretty clean too, which isn't always the case with towbar mounted racks.
I can't believe I used to go to the hassle of taking dirty bikes apart and fitting them inside the car all the time, it's so much easier to put them on the roof. Mind you I get a bit nervous when I stop at the services, and don't leave them out of sight at all if I can help it, even though they are locked on, and I'm pretty careful to make sure no-one is following me home too, so they do have downsides, not in practical terms though, they couldn't be better.