I usually wear smalls in bike shorts/tri clothes, but I wear a medium in the cw-x tights.

I like them, but only wore out my first pair because my ginormous thighs rub the seams. Now I wear a pair of short compression shorts over the top of them.

I have a 3/4 pair for fall/spring and an insulator pair for snowboarding and winter running. My muscles feel more "springy" when I wear them.