"Anybody use sling-style yokes, what do you think of them?"

are you asking that about sling style yoke pads, or an actual sling style yoke instead of the wooden thwart/yoke?

I doubt your asking about a sling yoke, as you'd have to move the center thwart back out of the way to use one ( or maybe carry the boat reversed ?)

I have a sling yoke from Mohawk - 2" webbing across the shoulders,  and it works fine for my royalex Rendezvous for getting the boat to the putin or takeout.  I don't think I'd want to carry a heavy boat over a long portage with it.  There is also a bit of flex in the hull wit each step, as the weight of the boat "increases" with the downward momentum of each step - while a composite boat is stiffer, that tendency to flex might cause some cracking in the gel-coat or clear coat finish ?