Works great. I have a GTX 285 in my Mac Pro and it works fine. I did notice that Xbench reports that OpenGL is a little slower, but it didn't seem like a big deal to me.
I have the GTX 285. Once I installed the upgrade I got a kernel panic. The first thing I tried was removing the graphics card and replacing it with the stock card and once I did all was fine.
I have not tried a "clean" install but I did have Leopard installed with the latest GTX 285 driver already installed. It worked fine after Snow Leopard finished installing. I never had to remove my EVGA GTX 285 from my Mac Pro.