Mike Bean wrote a review of Thule Sidearm Universal Bike Mount on July 27, 2011
This is about as good an option as I've found for roof-mounted racks. Super easy to use (once installed), and incredibly stable. Takes about 15 seconds to load and/or unload a bike. Dropped a star for two drawbacks: if you want to mount it on the driver side you have to spend 30 minutes swapping the whole system around (minor annoyance), and even locked all a thief has to do to take your bike is disconnect the front wheel. If you have to leave a bike in the rack unattended in a sketchy area, I strongly suggest a heavy chain or a U-lock.