nVidia's driver support is the best around bar none IMO.
They have drivers for just about every OS out there, including OS/2 and BeOS.
Their Linux drivers are, by a very big marigin, the fastest around when compared to their Win9x/Win2K drivers.
And finally, their OpenGL drivers are unmatched in the commercial market.
Some people have had alot of problems with some driver releases though, but since Im not one of them, I cant really comment on how thats been dealt with.
Then of course there are some people who get PO'ed at them when the latest beta drivers dont work, which is, IMO, rather moronic.
I was considdering a Radeon, but decided to wait and see how ATi's support turns out this time around, if they do well, Illl trust them the next time, if they dont, I'll be glad I didnt this time.