I would consider the GTX 650 the gold standard for what the Nouveau drivers could do, which is pretty decent. Consider that Nvidia is the only driver to come close or even beat it's Windows driver, that's pretty impressive on Nouveau's part. Intel and AMD are still behind their Windows driver, and AMD's open source drivers are behind Catalyst, although that gap is closing.
If Nouveau got the DX9 state tracker, I could see a lot of people wanting to use that driver over Nvidia binary. Given that reclocking works fully on most graphic cards.