I've been using the Louis Garneau one. It's not super thick, but it's kept me reasonably warm in the low 20's without too much problem. It's a bit small for my giant head, so it's not super form fitting when I pull it up over my nose (my nose and chin don't fit in quite the right spots (and it can result in a lot of exhuast fogging my glasses. Still, my neck and ears weren't at all cold (like another poster though my top sustained pace is probably 20 or below and my commutes are only in the 30-40 minute range). I so far it's cut the wind enough that if I want more warmth in my head, I'll just throw on my waterproof helmet cover which turns my head into a furnace.