I’ve never used one, but I researched the suction-style racks extensively and overall, they seem highly reliable. That said, there’s a part of me that would always wonder how secure it really is, and when driving I fear I’d spend more time looking in my rear-view mirror making sure my bike was okay rather than actually watching the road ahead, so I went with a standard rack.
However, I have to say that this SeaSucker commercial from a few years ago is one of the best commercials I’ve ever seen 
(It still shows up on this year’s TdF commercial breaks from time-to-time):
