They are waterproof... And they seal really well around the leg, so it's hard for water to get in from the top. I've been wearing them for the stream crossings on our recent snow rides, and they have kept my feet dry, even without waterproof shoes. The drawback is that they need an undercook for the really cold rides, and that setup might not fit in a tight shoe. My Pearl Izumi have a pretty good sized toe box.
I bought the Gore "socks" (more like wetsuit boots) to try, because I found them on sale. I'd like to pick up a pair of the Showers Pass for comparison. Eventually... waterproof mountain bike shoes.