Olli Ries, one of the developers at Canonical that's long been involved with the Mir display server efforts for Ubuntu, has blogged about using Unity 8 with Ubuntu 14.04. The Unity 8 session is still very much considered in an experimental/preview state and much of it doesn't work yet, including bringing up the session on non-Intel hardware might yield problems. However, there are some Unity 8 Qt QML apps available brought over from Ubuntu for phones, terminal support, and other features that can be toyed with for Linux enthusiasts and developers.
Unity 8 was a disaster last I tried, but that was a while ago so I look forward to trying out now with Ubuntu 14.04 LTS along with the performance state of XMir. Olli's Unity 8 preview information can be found via his personal blog.