I use a GeForce 6800 (nvidia), and love it. I can't really compare w/ATi, as most of those I've ever used were in (elder) macs, and pretty old.
I use nvidia's driver - unlike some other folks, I find it pretty easy to install. I never had any trouble until I went from fc5->fc6. In order to use the FC6 aiglx/compiz, I had to run the beta nvidia driver for a while. The latest nvidia driver enables the fc6 compiz/desktop effects.
Chris
