Counter anecdote mode: engage!
As an owner of many Nvidia cards, I find they crash infrequently, and when they do it's because:
* I was running Windows 8 Preview and pre-release Nvidia drivers
* There was some sort of hardware problem with my systems
I have used four ATI/AMD graphics cards over the years. Not all of them were negative experiences, but none of them were relatively more positive than using Nvidia hardware.
Before that, I had an ATI HD 5870 for over a year. I had about 3 crashes on the desktop, some that lost me work. The anger from losing work is what I remember most strongly, and made me vow not to buy ATI next time.
Before that, an 8800 GTX. I had 0 crashes from the desktop over several years.
I was running Nouveau drivers on the 670 for no more than 60 seconds before it crashed.
Verified on three machines with identical hardware.
AMD doesn't have decent drivers even for Windows. (By decent, I mean less than 1 graphics-related crash on average per year.)