I have an Allen rack also, and it's nice and steady, although I've never used my CF bike on it. I found it does leave rub marks on the car in a few places, but that was a non-issue since I was only mounting it on my winter beater car. Your needs may vary if you don't want any marks on your car.

Eventually I upgraded to a hitch mounted rack, which I feel is a) better for the bikes and for the car and b) more secure on longer drives.