Works like you think it should
Just purchased and have not used many times, so I can't comment on durability although they seem sturdy. I bought racks to carry two kayaks on top of a 2008 Subaru Outback 2.5 XT using factory aero bars and everything fits fine. Current kayaks being carried are 13 footers.
These things work as advertised, allow you to secure the kayak with confidence. With padding and caps on the J-cradles and rubber bumpers on the tie-down buckles, they are easy on the boat.
Key Pros: Easy to install and remove, easy to secure kayaks (the included tie-down straps are nice and keep you from having to tie a ton of knots)
Some Cons: I think they are a little pricey given their simplicity, and there's no security measure to keep the kayak carrier on your rack (if that's a worry for you, you'll have to come up with your own way of cabling things together and then locking to the car somehow).