Quite a bit, I guess? I play a number of games, but no idea if they are "properly ported".

I do research into rendering algorithms and that sometimes involves straying from the trodden paths. This means that I always get sceptical when I need to use a feature that is a bit more obscure. With nVidia, I rarely encountered bugs in the driver.

I know for a fact that the Mesa software renderer sometimes lies about what it really supports amd does something different and completely inappropriate under the hood instead. A good example is that Mesa claimed to support floating point textures even if the support was not compiled to avoid patent licensing issues. Instead it would use 8 bit per channel internally, which totally broke some algorithms. I had to find out the hard way because the values I read back were suddenly horribly quantized.

What I want to say with these anecdotes is that "being able to render games so that they are playable" just shows that the driver quality is passable. A great driver needs to get a ton of additional things right that are not commonly used for games.