Thought I'd mention that I tried my new Thule sliding roof bars over the weekend. They are really good and allow you to put stuff (bikes in my case) on and off the car without the risk of dropping stuff onto the car or worrying about the paintwork when leaning over the car etc.
Much easier and worth a look if you are buying roof bars.
They slide out from both sides too which is useful! They don't stop all the crap from falling of the bikes and onto the roof though

thought I'd share.....