We use Thule straps to secure the yaks to the racks, and bow and stern tie down lines for extra security. It does take a little time, but worth it to know the car and kayaks will safely get to the launch point.
We also have a set of Yakima cradles that we bought before the Thule set up. Good, but not as solid as Thule.
Bottom line: a little pricey, but worth it.
I would recommend for anyone looking to move their kayak without having to top mount it on cross bars.
I would opt for a lower profile kayak rack if I only needed to carry one kayak.
They are made to universally fit all bar shapes, which means they do not firmly lock on to the regular square Thule bar profile. When you try to reposition a heavy boat they tend to rotate on the bars. In addition, they are super tall so even my little Audi wagon has trouble fitting in an average parking garage. Gas mileage also takes a big hit. I ended up taking them off and threw them in the corner of my garage and went back to my 20 year old saddles. This is the first piece of Thule gear that I have been disappointed in.
The new version that folds should be better. I may give those a try...