I am on Ubuntu 16.04 (Xenial Xerus) and have heard that, with this distribution, I would be able to get a better look at the Unity 8 and Mir desktop interface. I understand that it is still buggy and in development, but I have read a lot of good things about it and wanted to try it for myself. The laptop I run Ubuntu on has an NVIDIA Geforce 940M and has the latest graphics drivers from NVIDIA (so they are the proprietary drivers). When I first tried to install and switch to the Unity 8 desktop, I was greeted with a blank screen followed by a mostly frozen login screen where I could do nothing but suspend or shutdown my computer. Tried reinstalling, making sure I followed the directions to the letter, but with no avail. I decided it was probably that the proprietary driver I was using were not yet fully supported, so I switched back to the Intel Nouveau drivers, rebooted, chose Unity 8 instead of the default desktop, and was met with a blank screen this time. Not even a frozen login screen, just completely black. Is there no way I will be able to test Unity 8 on this computer, or is there some trick with NVIDIA graphics and/or Nouveau that I have not been able to find yet?
It seems the next version of mir (v0.25) will support nvidia.
See bug #1628478 at https://launchpad.net/mir/+milestone/0.25.0.
The bug milestone was changed to 0.25 from 0.26 a few days ago which makes me think that much progress was made and hopefully an 0.25 release is imminent.
To test Unity 8, it requires Mir support, which is currently only partially implemented in the proprietary Nvidia drivers and at this time not enough to run Unity 8 / Mir properly.
In my case I installed anyhow (just to see what happens) according to this link and what happens in my case, upon selecting the unity symbol at the login screen and then selecting Unity 8, is that, after entering my password, my screens (3 of the same connected to a GTX Titan X) turn black briefly and then show the login screen background again with the top status bar that you see when logging in on Unity 7, but no menu on the left and windows / alt keys don't bring up anything. So basically you can't do anything (AFAIK) but to select the power icon in the status bar at the top-right and "shut down" and then "reboot" - at which instance my computer switched 2 displays off and only its "main" display switched to standard text-based console mode with the reboot image with the dots showing and cycling through repetitively but that's where the process hangs, keyboard doesn't work either, so I had to do a hard-reset.
Hopefully Nvidia updates the Mir portion of the drivers soon to the point that Unity 8 works, I can't wait to test it out, looks very promising! (hopefully by the time Ubuntu 16.10 comes out in Oct!)
I feel your pain when it comes to test unity+mir with proprietary drivers. Since the previous answers mentioned why and when mir will work with nvidia driver, I would like to add that one can test the next version of mir before it comes to the stable repositories. You may add this ppa to test next version of mir (currently 0.25+):