14 DAY TRIAL // Canonical has a very dedicated team building the Mir display server, and they are updating it on a constant basis. They have just released the 0.7.0 version and they are getting closer to a stable version.
The Mir display server is still available only on the Ubuntu for phones platform, but the developers are working to bring it to the desktop. It will be a while until that happens, but in the meantime we can see what is changing and what is being implemented.
According to the changelog, the mechanism to override Mir client functions has been added, the fd resources are preserved until the end of the sending of the message, various improvements have been implemented to the SessionMediator test, the integration and unit tests are now linked against the server objects, and the motion event resampling and prediction have been enabled for a more responsive touch experience.
A complete list of improvements can be found in the official changelog. You can download Mir Display Server 0.7.0 right now from the official website, but this is the source package and regular users can't really do anything with it.
Remember that this is a development version and it should NOT be installed on production machines. It is intended for testing purposes only.