The first time I played this I was on a Half-Life mod playing binge. There was Natural Selection, Science & Industry, and Knights, Pirates, and Vikings (may have messed up the name sequence on the end one there). They all had their moments, but, out of all of them, NS had the most visceral experience.
The first few times I played it, I ended up as one of the marines, and we got annihilated. It wasn't until a veteran commander came in that we started to win. He had a mic on, and started barking out these orders and everyone just hupped too. The aliens didn't stand a chance. It was the first encounter with true online co-operation. Strangers working together fluidly and without squabbles. It was beautiful to behold.