I just upgraded my main workstation, Cloud (named after the famed FF VII character with the big-ass sword) to use X.org.
Was a mostly painless upgrade. Didn't even follow existing instructions on the upgrade; I just noticed that a dist-upgrade grabbed a lot of xorg packages, so decided to take a look at xserver-xorg.
Initial problems: couldn't get OpenGL to work right. That didn't take too long to fix, though I'm not sure whether it's the refresh of the configuration I did or today's dist-upgrade that fixed it.
Second problem: it didn't really want to upgrade my XF86Config-4 file. Solution: nuke it, reconfiguration the package, and let debconf do the work.
There were a few other annoyances (mostly related to the dga and xv extensions that are never installed by debconf, for odd reasons), but it's been extremely painless. The main remaining annoyance is packages that still insist on linking on the original version of the OpenGL libs (such as xscreensaver-gl or doomlegacy); those won't install. But such is life on the unstable tree.
A lot of people reported better performance, but I haven't noticed anything. However, the radeon driver has always been very good even in XFree86, so that may explain it.
The main positive is that I'll finally be able to counter taunts from the Arch Linux fan at work.