I bought a pair during Spring 2011 and they quickly became my go-to short for hard rides in Arkansas's summertime heat. The compression keeps you feeling fresh, the fabric breathes well, and the contoured chamois pays off when rides stretch into the 2-3 hour range.
They're definitely cut for a performance (European style) fit, and the medium gave me just the right amount of compression at the legs without being overtight at the waste (I'm 5'10", 31" waste, 150 lbs). If you're used to wearing an American brand like Pearl Izumi, expect these to be quite a bit more snug and consider sizing up.
My only complaint (and this is entirely my fault) is that I bought the white-paneled ones and they were a bear to keep clean. They do make you feel fast, but be super careful changing a flat because any contact with chain lube or brake grime and they'll never be the same.