Canonical has a very dedicated team building the Mir display server, and they are updating it on a constant basis. Version 0.0.8 has just been released.
Ubuntu 13.10 will have the Mir display server as a default boot option but, until then, a lot of work and time will have to be invested in this technology.
Highlights of Mir Display Server 0.0.8:
• Surface lifecycle notification has been added to the session listener;
• GLPixelBuffer has been moved to the shell;
• ViewableArea has been replaced with more fitting interfaces;
• Unity armhf tests have been disabled.
A complete list of improvements can be found in the official changelog.
Download Mir Display Server 0.0.8 right now from Softpedia. Remember that this is a development version and it should NOT be installed on production machines. It is intended for testing purposes only.