I have two pairs of DeFeet Kneekers Wool that I like very much. I love that they keep my knees warm, and when the temperature even climbs up later, I don't have to remove them. I guess the wool is just great material for riding in Southern California. Good stuff!
Also watch out for knee warmers that can irritate the back of your knees. I have one from ProBikeKit like that. It is made of great looking materials, but it just irritated the crap out of the back of my knees with every pedal stroke.