Apart from 3d stuff I do alot of 2d stuff aswell as some video editing, and Nouveau is perfectly competent for that. No, I do not play typical AAA games, but most of the indie bundle games I've played have ran just fine, though some of they have had graphical glitches. Was quite a while ago though so I should probably check to see if those have been fixed by now.
And as for the performance differences, as has been repeated over and over again, Nouveau operates at the lowest clock frequency while the proprietary drivers re-clock at higher frequencies when you are running 3d intesive applications. The Nouveau devs are working on re-clocking but since this can be cause damage to your card unless done correctly they can't enable it until they are sure (within reason) that it actually works.
Most definitely the proprietary driver will still come out on top and by a good margin, and for those who wants/needs the best possible performance it's great that the NVidia driver exists, for my needs Nouveau is already adequate, and with re-clocking enabled sometime in the future it will be even more so.
And finally one more thing, while Linux is 'blessed' with proprietary driver support from the likes of Nvidia / AMD, there are other platforms which will NEVER have any official support, one of those which I'm very fond of is Haiku, and here Nouveau serves a huge purpose as a possible means to support NVidia 3d hardware acceleration in the future.
We all would like to have open source Nvidia drivers, because they make great graphics cards, but this is not going to work.