During the LAN party era my friends and I played the crap out of that series. The engine wasn't as sophisticated as what iD et all were doing, but worked perfectly fine for the gameplay. We'd spend hours trying to one up each other in deathmatch.

Another gem of a game I think a lot of people overlooked that has some similarities is Natural Selection 2. This is a sort of aliens vs space marines FPS and RTS hybrid. It never really blew up but still has a community.