Tried the Conti GP5K - tried to like them, but any kind of hit to the sidewall (rock in the road, or riding on gravel) would destroy the tire. After having to discard two sets of these, I gave up on them.
I have since switched to the Conti GP 4-Season and have been very pleased with this all-rounder. Still a nimble tire with good grip, but much more versatile with respect to road condition and durability than the GP5K.