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, internationalization, etc.
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. Options are listed with:
$ egmde --help
There's more about customizing the appearance of egmde on the Ubuntu discourse: