I have cards of both brands, and it depends.
1. HD4670 512MB GDDR3 in my Media Center. Works fine, but I do get screen tearing when watching Youtube videos. Plays games fine. I haven't tried many games though. Only Crysis Warhead, Left 4 Dead 2 and one or two more.
2. HD4550 512MB DDR3 I put in the computer I built for my mother. Perfect for the kind of work she does and it has Flash HD hardware acceleration. Hasn't given a single problem yet. Of course, I don't update drivers often as there is no need to.
3. HD4650 1GB GDDR2 AGP on my legacy Pentium 4 3.4 Ghz Northwood system - drivers on the CD install fine. Latest AGP hotfix sends your computer into an endless reboot cycle unless you start in safe mode and wipe everything AMD driver related with Driver Sweeper. This happens to many people. AMD does not store previous AGP hotfix drivers unlike they do with regular drivers, which means you are either stuck with a driver from 2009 (the CD that came with the card), which does not even provide HD flash acceleration, a must feature for what is essentially more of a media card than to play heavy duty games, or you go hunting online for another version. But first you have to go through the forums to understand what is the latest driver that actually works. Turns out the latest is 10.10 for some, 10.9 for others. Google it, download from third party site. Install. Install goes fine but CCC does not load and sometimes keeps displaying a continuous error message that it stopped working. You can't reinstall old CCC. You can't uninstall new CCC alone. Must uninstall everything. Install from driver CD. Install new driver (only) on top of older driver + older CCC. Works fine afterwards. But it's a mess to get there.
4. HD6850 1 GB GDDR5. Owned it for about a month. Kept hard crashing the PC (had to manually shutdown) in certain games - Crysis in certain levels for example after a few seconds (other games would run well for hours, and so did benchmarks - although some people crashed on Far Cry 2 bench). Turns out the card was defective, as many others were - there was a problem with voltage spikes regulation on a batch of cards from AMD - many brands involved. Very hard to reproduce, especially since stores and manufacturers don't test with normal games. But I found out about the cause of the problem two months after switching to a GTX460 1GB.
Had driver problems too. Mouse pointer got corrupted and then self corrected after a while without explanation. Happened to other people with other card series and drivers, so it was probably not due to the card having that particular fault that made it crash in certain games. Certain Youtube videos would lock playback after a given time and you couldn't playback what was left of the video even if you reloaded the page.
When the HD68xx series was launched, AMD released Catalyst 10.10, and then non supported Hotfixes 10.10a, to correct the simple fact that Windows XP froze with 10.10 drivers, 10b, 10c, 10d and 10e, in a matter of days. Then 10.11 didn't support the HD68xx series. People had to stay with an unsupported 10.10e if they wanted the features they paid for.
The following didn't happen to me, but it is a fact: when the HD69xx series was launched, AMD had just launched Catalyst 10.12 two days before. Yet those drivers didn't have support for the new cards, released two days after. You had to either rely on drivers from the CD (don't know if it was the case, but they may be Release Candidate drivers - the ones in my HD6850 driver CD were RC drivers, and so are the drivers on Gigabyte's website for the HD4650 AGP), or the 10.12a Hotfix - again unsupported, non WHQL drivers.
At that time, I even took notice that AMD changed the wording on the legalese disclaimer on their Hotfix driver download page to something a little less clear that directed you to read the full terms - and i read them and there wasn't anything related to that...
Geforce 2 MX400 (also have an MX200 from an old computer that was given to my mother once), FX5200, 6200, 7600GS, 8800GT, GTX460 1GB, GTX480
Never had major driver problems.
I do however have a recurring problem with drivers past 258.96, in that none of them are able to handle my monitor through the D-sub port properly, unless the monitor is at it's max resolution. The monitor also has a DVI port but it never worked properly, so I'm stuck with VGA. So, I'm stuck with 258.96, but it works very well, first with the GTX 460 1GB and now with the GTX 480.
There are of course facts: when Windows Vista was new, a big chunk of BSOD's were not Microsoft's fault, but mainly GPU drivers, with Nvidia leading the way.
Once they released a driver that had the fan profiles messed up and some people ended up with dead cards because of overheating.
All in all, I can only speak for my personal experience, but I think I can say these two things:
1. Nvidia doesn't release too many drivers (certainly not one a month, unless it's justified), they haven't even touched their control panel in months (and I couldn't care less, as long as it's functional, I'm not playing a game called "Control Panel", so...), but when they do release a new driver, it tends to be more solid (despite those serious problems I quoted).
2. AMD probably doesn't do so much of an extensive testing - after all they release drivers every month, and from the forums I visited, it seems now and then they brake something they had already fixed before. Some problems seem to persist for months - some people keep complaining about corrupted mouse cursors (which in all honesty may not seem like a big deal, but it is as it disrupts your work completely), and glitches/problems in some games. My guess is they have a much smaller array of test systems and therefore let slip some incompatibilities - if you add that up to the speed at which they release drivers, it ends up in weird problems. Of course, if you happen to own one of the test systems they have (I presume you will be better if you have an AMD based system), you are lucky and will probably almost never experience a problem.
All in all, lots of people are fine with both brands, really, it all depends on what you have, what you do with it, and whether you buy a brand new - just released card or not.
Rule of thumb: in the present conditions, if I had to buy a brand new, just released card I wouldn't buy an AMD one. Chances are Nvidia has a properly tested WHQL driver on their website and AMD doesn't which doesn't mean Nvidia drivers don't have bugs, of course). AMD is however working on a new model of driver, which includes everything related to the AMD platform (if you have one), and they are rewritting their drivers to take advantage of the new HD69XX series, so after all this turbulence is past, they might end up with a nice consolidated driver structure. Time will tell.
Edited by tpi2007 - 4/26/11 at 4:50pm