Canonical has a very dedicated team building the Mir display server, and it is updating it on a constant basis. Version 0.0.13 has just been released.
Ubuntu 13.10 will no longer feature the XMir display server as the default boot option, but users will still be able to download and install it from the official repositories.
According to the changelog, a compiler warning about maybe-uninitialized struct member has been fixed, client side support has been added for connecting via an existing FD, a missing directory separator is now present when searching for a config file to parse, and much more.
Remember that this is a development version and it should NOT be installed on production machines. It is intended for testing purposes only.