The Unity 8 Desktop (Preview)
The Unity 8 Desktop Preview is the new Unity 8 desktop shell running on the Mir display technology. It's called a preview, because it's a pre-release view of technology and designs that are not yet ready for general consumption as a full desktop experience. It's there for developers, and anyone else who is interested, to test and review how the Unity 8 stack works in the desktop experience with an eye to full Ubuntu convergence.
Getting the Unity 8 Desktop Preview
The Unity 8 desktop preview is available as an add-on package. To install it, open a terminal and type:
sudo apt install unity8-desktop-session
After finishing the installation, simply log out and then select the Unity 8 session at the greeter prompt.
What to test
Try installing applications, using things, check out the store and scopes, etc.
Unity 8 uses Mir display technology. Proprietary video drivers do not yet offer the required level of support for Mir, so your experience if you're using the binary block nVidia or AMD drivers will be somewhat disappointing.
Issues should be reported on launchpad, using the appropriate component. In doubt of what component is to blame for the issue, file the bug against unity8-desktop-session.