Alex gave a very interesting talk at KVM Forum 2014 about the current state of VGA passthrough using KVM & VFIO:

Also, I think nVidia is making an incredibly silly choice (apparently accidentally) causing Code 43 in their drivers when virtualization is detected and refusing to fix the bugs. Virtualization is becoming evermore powerful and this is just going to push potential customers away to AMD. Once they establish a reputation for their cards not working well with virtualization, they're going to have trouble gaining custom confidence even if they reverse their stance on not fixing the Code 43 bugs.