I have been using nVidia cards on Linux since the GeForce 2. I have never had any problems with their driver (apart from misconfiguration by me before X.org started autodetecting screens/GPUs and xorg.conf etc. was deprecated).
It's a small thing, but their binary drivers do not support kernel modesetting, so if I want to switch between X and the console it takes several seconds with a visible screen blank. Using nouveau (or anything that supports KMS) is very fast by comparison.
