Good quality warmers, with excellent silicone grips on the inside and outside of the top, and a snug zip at the bottom. These didn't slip an inch! They're not very thick, so great on a crisp winter morning and weren't too hot when the sun came out either.
I'm a long-legged 6'3" and went with the L, not the XL as others recommended here, and the L were absolutely fine - with a couple of inches to spare under the shorts. Any larger, and they would be too loose - so if you have long chicken legs, go L.