I had to turn to Nvidia after 3dfx went down, and I never liked them. I had GeForce3, 4, and an FX5600.
It always appeared to me that the Nvidia cards lied about their framerates. I remember switching from my Voodoo5 to a GeForce 3 and doing timedemos in quake3. The GeForce always beat the 3dfx in average fps, but running around the same map I would experience noticable slowdowns in certain spots where the voodoo didn't. I was very unhappy with this until I got my GeForce4.
The last slap in the face from Nvidia was when I upgrade (heh) to the GeForce 5600 Ultra. This card is a serious joke, and runs terrible in almost every DX9 game I've played it on.
Since then, I have upgraded to a ATI 9800XT. Very expensive card, but it looks so much better and runs way faster. My fps went up about 300% switching to the 9800XT vs the Nvidia 5600 Ultra.
I've also BBR'd a 9600XT for testing sake, and it was about 40% faster in most of the DX9 games than my old 5600 Ultra.
Nvidia is old news in my opinoin... Maybe they took on too much dead weight from 3dfx.