Er, I find it pretty fun. Fast learning curve and quake style play.
http://www.alientrap.org/nexuiz/
^their website^
Incredibly fun! If you're too lazy to buy Quake, this is a good alternative.
And great graphics for a mac freeware with low requirements. If I get 70 FPS anybody can.
Downloading...
My RL friend downloaded this a few years ago, and told me about it, but I didn't really find it that appealing.
Be sure to watch the video on the page I linked to, gameplay is like that.