I'm 6' 175 and 34 waist, I have the Larges and they fit perfect. I tried on a set of M and they fit tight, but the crotch would fall down.
I use these every day for commuting on my bike in the cold weather, I'll wear just these in temps down to 35 degrees then I'll need to put on a softshell pant over it. I also used these multiple times skiing which works great as an underlayer, they are just as warm as my HH warm unders but offer much better support and recovery.
I also have the CW-X "Pro Tight" and these actually block the wind and are way better for cold weather. I'l l wear these between 50F and below, and the pro tight above 50.
They are made pretty burly, like the guy below said, they are built to last, and at $125 they are worth every penny.