I've been a more-or-less happy user of nVidia products for a few years.
I've enjoyed great performance from their cards & drivers, but I will
warn you that they are a liability when it comes to debugging a
misbehaving 3D app. I'm currently working with an application that uses
the OpenGL shaders intensively, and I've been having problems with its
performance. Its other components check out okay, so my suspicion is
that there's a bad interaction between some component and the nVidia
driver. Alas, I can neither confirm or dismiss my suspicions easily,
because the driver is closed-source.
Nevertheless, I'll continue to use those drivers until the day Linux can
claim FOSS high-performance fully OpenGL-compliant drivers for a modern
video chipset. I do wish that ATI and/or IBM would get on their horses
and resolve this problem. For the record, I may recommend a
closed-source solution, but that doesn't mean I have to like it. :)