Simple, secure, and cost effective.
When I got kayaks I was trying to avoid racks like the J-style Hull-a-Port. I've had issues with boats shifting, and am not a fan of the design. The Stacker is a simple design that uses the crossbars to help keep the boat in place. Never had an issue with shifting, even on 8+ hour drives with two boats. It's also the cheapest Thule option for more than one boat. Very happy with my decision. Just make sure you put the straps on BEFORE you put the boat on.