Running nouveau here on my G92 8800 GTS 512MB, have been ever since it's been available on Fedora, and now running it on Kubuntu also with no problems whatsoever the entire time. It may still be beta in terms of features depending on the card, but stability has been rock-solid for me pretty much the entire time. Still running plain xf86-video-nv on my other distros for the time being, but I've mostly moved from Gentoo to Fedora as my main distro now.
I use Fedora 12 with provided nouveau driver on my pc (7600 GS AGP) and my laptop (8600M GT). 7600 gave some initial problems (X randomly crashing) but they were promptly solved with updates, now it runs solid and stable. 8600M never had a problem with nouveau, I just wish for some power management as laptop cards tend to be overkill for batteries.
It used to work quite well a month ago, then, I updated it and I lost the suspend to ram (doesn't wake up). Then, after this merge, the computer doesn't suspend at all and the only 3D application working is GlxGears.
I'll report all this by the end of the week.
However, I'm really impressed by the work they've done, may they keep the good work on !
I currently use a NVidia TNT2, Nvidia Quatro 140M, and a Nvidia 6600GT across my different machines. I'm not to worried about the TNT2 though. The nv drivers is good enough for me. That is just sitting in a server with only text mode enabled.