Apples new OS has hit most stores by now, and it claims to give a large performance boost to your Mac. When it comes down to Nexuiz (or OpenGL in general), performance is much worse than in 10.5.8. Phoronix did a nice little benchmarking test, where Nexuiz' FPS was halved by upgrading to 10.6.
Let's hope Apple fixes these problems soon with an updated build, but if you like to game on your Mac - you might want to postpone the upgrade until that fix is out.
On my old macbook I also ran the bench1.dem in Snow Leopard / latest Nexuiz SVN and got a grand total of 3.33 FPS.