I give this a 6 on the basis of it being a great idea -- hopefully there are people out there for whom it works better than it has for me. I have to load and unload my kayak on my own, so I'm on the lookout for anything that might help. This one, so far, has not.
Part of the problem is that I have a great Yakima bike rack on the back of my car, so I can't use the back window for the Sherpak and loading my kayak. But even when I tried using the side of the car (the Sherpak can be angled in two different ways), the weight of the kayak knocked it off the car windows. And I seriously DID wash the windows first, so that should not have been the problem.
I may try it one more time but, unless I have much better luck, I'll be returning mine.