Ditto to all the other reviews below - love these tights. I've used them for all of my marathons thus far and cannot say enough about how supportive they are.
Just wanted to add the reference that I'm 5'2" & 120#. I typically wear a size 4 or 6. My mid section is trim, my bum is pretty average and my thighs (21")/calves (15") are monsters. I'm in the grey area for xs or s, so I ordered both to try on. When they showed up, I didn't even try to wedge my calves into the xs...I went straight for the size small. They were perfectly tight when I started but stretched out a touch after the first 50 miles or so. They still provide support to my knees, but I perhaps should've gone with the XS for more support. Grey area is grey area, I guess.