So far I've run some benchmarks seeing the 2D/3D performance impact of Unity, GNOME Shell, LXDE, Xfce, and other desktop choices paired with their newest versions packaged for the Ubuntu 14.04 "Trusty Tahr" plus its updated graphics stack with the Linux 3.13 kernel and Mesa 10.1.
Everything went well with that testing before deciding to get into the unity8-desktop-session-mir and unity8-desktop-session-x11 testing. When installing those packages on Ubuntu 14.04 and logging into the environment, it was a complete mess... Swipes didn't seem to work, the UI was barely responsive, I didn't find anything amazing out of the bottom edge, and it was just an unusable mess at least when trying it out from a laptop. Granted, watermarked across the screen is "EARLY ALPHA NOT READY FOR USE."
The Unity 8 Mir version wasn't any different from the X11 version but coming up soon should also be new XMir vs. X.Org Server native Unity 7 benchmarks for those interested -- the previous tests were from early January on older versions of the desktop and kernel/Mesa. Just passing along this information for anyone else that may be tempted to try out these experimental Unity 8 packages from the desktop on Ubuntu 14.04 LTS.