I bought the arm and knee warmers together at the same time. Check my review on the arm warmers for all the superlatives as the knee warmers give the same level of excellent warmth and wind protection.
My only disappointment with the knee warmers is the sizing. I'm a stocky sprinter-type and needed a L/XL to go over my thighs comfortably. My inseam is 31" so these turned out to be the right circumference but the wrong length. They look more like hybrid knee/leg warmers on me. I could pull them up higher but then I get a bit of extra material around the knee.
If you're taller than 5'9" then the L/XL size should suit you perfectly.