I played Quake Team Fortress semi-religiously in college. From Friday night to Sunday morning, the engineering computer labs were dominated by people playing it. I kept playing it up until John Carmack open sourced the Quake code and everybody started cheating. Then it wasn't fun anymore because I'm not everybody.
So I must say that the teaser trailer for TF2 looks most excellent. It looks like the characters are all on a revamped Two Forts map, i.e. the map everyone used. Great. Someone might get me to spend my money on a First Person Shooter soon. That hasn't happened in a while.