I've got a Mont Blanc Roof Rush and a Thule ProRide 591.
The Mont Blanc is adjustable enough to take a Child's bike (Isla Bike Beinn 20) which the Thule wouldn't and I think it is much easier to use.
The tyre channel holds the bike upright without extra support whilst you put clamp in place. One lock locks both the bike to rack and rack to roof bars, when empty it can also lock the clamp in the lowered position, I only found that out the other week. The Thule needs a bungy cord when empty.
A downside to Mont Blanc is that it only takes up to a 2.2" MTB tyre, there is another model which takes a wider tyre but I can't find any suppliers online.
Compatible with most brands of roof bars, steel and aluminium. Holds one bike. Locks both bike and cycle rack with one lock. Hands free securing. Award winning. Versatile clamping arm means that the carrier can be changed from left hand to right hand allowing it to be mounted on either side of...
www.montblancgroup.com
It's worth having a look on eBay as car dealers sometimes sell off bike racks and they are usually Thule or Mont Blanc in a different box. My Thule rack was from a Nissan dealer and Mont Blanc from VW.