They don’t cover aspects of a desktop environment that are not related to using Mir. Missing functionality includes: integrating into the system for screen locking & suspend, policy kit integration, and internationalization.
To use egmde as your desktop, select it when logging in, to run it on your desktop run "egmde".
Egmde can be configured by editing ~/.config/egmde.config or on the commandline. Common options are documented in the file created by default. All options are listed with:
$ egmde --help
There is also some limited support for "remote desktop" meaning egmde can be used over VNC.