X.org still remains harder to test than the kernel and part of the problem is the scattered repos. There are scripts that will build you a local xorg from git (without killing your working system xorg) and I have actually managed to successfully complete a build that ran. Unfortunately the build would crash as soon as I pressed a key and I eventually had to give up after banging my head against the wall.
The best I've managed so far is testing development versions of the intel driver by creating custom ebuilds and testing on my X setup and reverting when I loose a working X.
The process has certainly improved over the last few releases but there is a way to go.