Love the stacker! I was hesitant when I bought it because it's meant for white water kayaks, and I own a 10' Pelican recreational kayak & 16' Perception touring kayak. I've used the stacker several times now and the kayaks not only fit very nicely, but are very very secure. The rachet straps that are supplied with the stacker work perfectly.
Of course because my kayaks are deeper than white water kayaks, I can only fit two boats on my roof rack (on a Jeep Grand Cherokee), but that's exactly what I wanted the stacker for. No more shoving one boat in the back & one on the roof :)