I disagree - I have a GTX660 and a mate has a GTX650TI Boost 2GB. I have other mates with HD7850,HD7770 and HD7870LE/XT cards. Linux is the only area you could say Nvidia is much better.
At least in the last 18 months AMD have been better
IMHO under Windows,as both of us with Nvidia cards have niggling faults.
Our personal experiences OFC.
In between our group,we own the better part of at least 700 games.
Among my issues:
1.)SR3 and SR4 would just go black screen with any of the 320 drivers. Solved by the latest ones. A problem which some people have had for longer with older Nvidia cards.
2.)Tombraider would crash unless I disabled tessellation first(and then enabled it once the game started) and I only read about the fix on another forum. Has not happened with the latest drivers.
3.)Metro:Last Light had weird tearing artefacts with the drivers at launch. Solved with later drivers.
4.)Planetside 2 was working perfectly too until I updated to the Nvidia "performance drivers" recently. The driver kept resetting itself and after that the game was a stuttery mess until I restarted it. I am not the first to have this issue and its been affecting some people for ages. Thankfully the latest driver,seems to have stopped this.
5.)Firefox black window crashes. I thought this was FF being rubbish. A forum member here pointed me to a forum thread which indicated it was a known driver issue. The latest drivers solved that issue,but I had it for yonks.
This was coming from an HD5850 which for most part(like another mate) gave me no problems,and only its tessellation performance when pushed was the cause for degraded performance(a hardware not a software issue).
My mates with the HD7870LE/XT,HD7770 and HD7850 had ZERO problems last time I checked in the last year. GCN based cards at launch did have their share of niggles though,but I certainly think AMD has put in more effort than Nvidia recently.
Plus going back even further. HL2 worked fine on my 9500 PRO but was utter crap on my mates FX5000 series card.
But OTH my 6800 series card had support for DX9c when compared to the X800 series which meant better image quality in games and so on.
Previously,my worst problem with drivers was when my HD3870 became a stuttery mess in the last level of Crysis even at lower settings and resolutions when compared to my 8800GTS 512MB when the game was launched(performance did improve with later updates).
Edit!!
PS:
We all play a lot of Blizzard games.
I would say we all seem to have been fine so far,and I would say that after 100s,maybe 1000s of hours their games between us.
Nvidia does supposedly have the performance advantage with them,but I have not noted any problems with the AMD cards with the games after talking to my mates.
Outside the extra performance of my GTX660,my massively overclocked HD5850 seemed to be OK running SC2 and D3 for example.
Its all swings and roundabouts,except Nvidia niggles get less exposure - look at how the $200million+ bumps issue got little or no coverage from the tech press,despite being easily one of the worst hardware issues in years. The tech press went all quiet over that.
I had to personally help out people who did not know why their laptops were dying,so they could actually stick it to the bloody OEMs,who were trying to cheat them by issuing poxy fan speed updates for the laptops.
If it had more exposure,many people would have pushed for getting their laptops replaced instead of letting it go.
Heck even the adaptive v-sync issues and throttling issues with Kepler were quietly hidden away. It took French and German review sites to expose the problems with Nvidia Turbo Boost,while the English language press barely mentioned it. Once AMD had different throttling problems,a big song and dance was made,since the sites got free cards shipped to them by Nvidia.
AMD has its own problems too,but they are more likely to be highlighted.