I bought a Thule 908 and used it for the first time on Friday, and its worth every penny.
So convenient its untrue. Expensive, yes, but a damn sight cheaper than the bike I'm trusting it to carry.
I went for the 908 over the others because of how easy and quick it is to fit, you literally place it on the tow ball and then push the lever down, thats it. It locks to the towball, the bikes lock to the rack, and its really stable.
Only problem is storage, as its a hefty piece of kit and weighs a fair amount.