My knees aren't very sensitive to cold, and I don't go down hills at more than 30 mph, so as long as it's above 35ish I just wear shorts. Colder than that, and I probably wear tights. (I'm not much of a knickers/capris fan for running or biking - if I'm cold I want tights, if I'm not going to stay cold, I don't want anything on my legs).

I love arm warmers though. Best invention ever.