Generally thick to increase durability - more so than a wool sock (in my opinion). In effect, you end up making your footwear 'smaller.' They don't breath at all. If you think a gortex shoe suffers in breathability, you might as well put garbage bags over your feet inside your shoe instead of wearing gortex socks. As your foot sweats it moves and brings the pleasure of blisters.
Realistically, however, I do find them beneficial in cold conditions as an extra barrier with lightweight hikers.