Perhaps overall, there are black spots dragging it down.
As an nVidia customer on Linux with a modern and high-end gfx card, I don't think I'm getting any worse performance that I'd be getting on Windows however (even when playing games via CX, which is a pretty nice miracle). I treat myself to highest gfx settings as well since getting the new card, too!
Ideally, I'm of the opinion that we shouldn't. That stuff should be ready and accepting for others to use, not to fix.
One of the benefits of OSS drivers is that 3rd party developers can debug bugs and performance problems they are seeing in their software and fix the drivers directly, rather than having to come up with hacks around it in their software.
But I agree that I certainly wouldn't expect Valve or anyone else to need to hire full time developers just for fixing Mesa. That can't be expected.