Canonical has a very dedicated team building the Mir display server, and they are updating it on a constant basis. Version 0.0.10 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 0.0.10:
• The Mir server socket can now be set with the MIR_SOCKET environment variable;
• The server-side tracking of client buffers has been fixed;
• A whitespace has been added to prevent errors in non -fpermissive compilations;
• A lifecycle events interface has been implemented.
Remember that this is a development version and it should NOT be installed on production machines. It is intended for testing purposes only.