Canonical is planning to bring the Unity 8 to the desktop, but it will take a while until this task is accomplished. Until then, users can test the new Ubuntu Next images, which incorporate Unity 8 and the Mir display server.
Ubuntu developers have been working very hard on the new Unity 8 desktop environment, but their progress has been limited so far on the mobile phones. With the work that’s being done for Ubuntu Touch RTM and Ubuntu 14.10 (Utopic Unicorn), the implementation of the new desktop is now on a back burner.
The goal of the Canonical is to unite the desktop and mobile experience and to have a single operating system that is capable of running on all the platforms. This will be done, eventually, but that's actually a long road.
The soonest that we might get Unity 8 and Mir working in concert in a Linux distro is with the release of Ubuntu 16.04 LTS (2016), and not even that date is set in stone.
Users can download and test Ubuntu Next on their systems, but this is a development version and it's not for daily use. In fact, you can't really use it for anything right now because it's basically one giant tablet.
In any case, check the gallery below to get an idea of what you might see if you run Ubuntu Next.