Great use of the Q3 Engine, also try Open Arena or Alien Arena.
PROS: FREE, modest system requirements (1GHz CPU, 512MB RAM, GeForce 2 equivelant GPU), runs on all three major OSes, can play downloaded user-created Q3A maps, very well polished graphics for a freebie, very diverse Nexuiz specific map packs available all over the net.
CONS: system requirements aren't as low as Open Arena (166MHz CPU, 64MB RAM, 8MB openGL GPU) nor Alien Arena (1GHz CPU, 256MB RAM, 64MB GPU), physics can be a bit exagerated, AI ain't as bright as Open Arena nor Alien Arena.
So, the pros definately outweigh the cons for Nexuiz. It's an awesome arena-based FPS that doesn't cost a penny. But if you're using a netbook or PC with a CPU lower than 1GHz (pre-Pentium 4), you may want to download Open Arena or Alien Arena (both also FREE) instead.