I used to work for a QA software testing company, and nVidia was one of our clients. We mostly tested console and PC games, as well as a variety of mobile software, but we had a dedicated team to testing nVidia GPU drivers and hardware. I almost never got to do any console testing, but because of my background and experience with PCs, they put me on the nVidia shiznit. I was also one of the few guys building the rigs we used for testing. We technically counted as nVidia employees, we had nVidia email addresses, and used a VPN to connect to nVidia's internal network. This was during the late 2000s. Those were some interesting times. I was on the night shift, and that's when I learned to love the night-time living lifestyle. We called the sun the "day moon." If it was in the sky, that meant sleepy-times. Anyways, I can't speak for ATi/AMD or Intel, but from what I saw and experienced, nVidia did/does put a good amount of work into testing the f*ck outta their drivers.
Personally, I as well as friends, family, and clients haven't really had too many issues with Radeon drivers over the years either. Catalyst was a step in the right direction, and Adrenalin got things to a pretty respectable place, I'd say.
My experience with Intel drivers has been limited to iGPUs on laptops. Can't really say much about that. So long as I could output to an external display via HDMI without issue, I didn't really require much else. Emulators and even some native PC games (old 3D ones like UT3 & UT2K4, and newer 2D ones like Streets of Rage 4) ran fine. The iGPU control panel software often leaves much to be desired, but then again you can't really expect it to be feature-rich, as there isn't much to the iGPUs to begin with.
