Quake itself was ok. The multiplayer version was fun. But the real fun started when people began modding the game. The original Team Fortress was actually a free mod for Quake which I'm pretty sure quickly became the most popular instance of the game for online play.

Funny random tidbit, I actually remember playing the game with one dude who specifically had to brag about having a high powered 1ghz processor as his username in the game (something like 1gigahertz or something cheesy like that). Pretty sure back then I was still rocking a 700mhz AMD Athlon Thunderbird processor.