I consider myself to be more "wishy-washy" on the subject than a fanboy, but I like ATi more after all of my experiences between the two.
A lot of how I feel about either camp is derived from first impressions with video cards which I have forgived, but can't forget. For example, my GeForce 2 MX had the worst color I have ever seen. I don't blame nVidia for that, and I do still buy nVidia (running a 6800GT right now), but I can't get over the gut feeling of high speed with lackluster color and overall image quality. My first Radeon, (AIW 32MB), blew me away with quality visuals after that GeForce 2, but the drivers back then absolutely sucked. I never got that AIW to work completely correctly. I spent days of my life playing drivers both official and modded trying to get that thing to work.
But then again, during the lifespan of my Radeon 8500, the overall performance of that card went up drasticly due to all of the improved drivers that ATi was putting out. I was also very happy with my Radeon 9800NP. I never really had any problems with it at all, of any kind. But I haven't had any problems with my 6800GT either, so /shrug.
Either way, I buy whatever card/brand has the best bang for the buck at the time I need it. I would never buy a lower performing or feature lacking card out of brand loyalty.