My first card was my brother's hand me down GeForce 2 MX. It wasn't much later that I wanted a bit more performance and I nabbed a GeForce 2 GTS off ebay because that was what my brother recommended.
This put me in the Nvidia camp out of habit. This was the peak of my Nvidia bias out of ignorance of the other side.
While researching a successor, I of course considered the GeForce 3 Ti500. This is when I noticed the Radeon 8500, and I considered switching things up. But I waited and the GeForce 4 Ti4200 came out and was faster. There was no bias in this decision, it was the better buy once it came out, albeit more expensive.
Now after the Radeon 9700 revolution, my next card was naturally ATI and I picked up the 9800 Pro. The GeForce FX series really gave them a terrible reputation. So much that I nearly blindly purchased my next card, the X1800XT. This was the peak of my ATI bias out of ignorance of the other side. I knew Nvidia had competing cards, but I did not consider them since I still had the FX series on my mind.
My next two cards were ATI/AMD due to good price deals. The 8800GT and 3850/3870 came out at the same time, and at the time I bought the 3850 for about 60% of the price the hotcake 8800GTs were going for. The 8800 revolution vs the 2900XT really hurt ATI in my view, but I simply couldn't afford the 8800GT. Just a year later, December 2008, I got a 4850 for $95.
While Fermi once again seemed to hurt the reputation of Nvidia, my next card was a GTX 460. I got a good used deal cheaper than the competing 6850 so price-to-performance won out. Then I shifted to a laptop for several years. It has a GTX 675M, which is a re-brand of 580M which is an underclocked GTX 560 Ti, underclocked so far it essentially matched GTX 460 performance. So I primarily gamed on Fermi for more than 3 and a half years and it never let me down.
But I still followed GPU developments and behaviors during this time. At this point I have a purchasing bias towards AMD because I strongly disapprove of many of Nvidia's business practices (Kepler, Gameworks, other proprietaries, GTX 970 lies). That doesn't mean I didn't notice AMD's framepacing issues early in GCN, Fury X pre-launch exaggerations, etc. but it is clear enough to me to draw a difference, and supporting the underdog plays a part (it's almost shocking that the now venerable 7000 series was somehow an underdog against the 600 series, imho).
My first dGPU since the GTX 460 was thus AMD, although I came very, very close to picking up a 980 Ti. I am more than willing to jump fence back to Nvidia pending they or AMD shift their courses. I'll gladly debate with people, but I do not let this alter my recommendations For most users and most price points either brand can be recommended due to their preferences and system. I'll never recommend a Fury X at current price points unless a user wants the best Freesync performance, for example.