running this now on my MacBook Pro Retina. i'm liking KDE Plasma 5.
it looks and feels very slick, with many noticeable improvements over KDE 4 (Kubuntu 14.10) on the same system. there are a few kinks to work out, but it's coming along very nicely.
a few days in, here are the biggest improvements i've noticed:
* Plasma 5 and the Breeze theme look and feel more crisp, clean, and presentable. the Plasma 5 desktop brings nice detailings, tweaks, and redesigns of many of the familiar elements of Plasma 4. in some ways, the renovation feels similar to the transition from Android 4 to 5 or Windows XP to 7.
* much faster login. it takes around 15 s, down from around 30 s with KDE 4.
* better handling of high-DPI displays. after setting the font DPI in System Settings (as the DPI of my display), Plasma and KWin appear to scale their icons and decorations accordingly. i haven't seen any breakage in the rendering of the system tray and panel widgets at high DPI (as there is aplenty in KDE 4).
* a proper built-in dark theme styled after the primary theme. Breeze Dark looks great everywhere, including Qt and GTK apps. it has somewhat better compatibility with older and non-KDE apps, such as Firefox.
* more useful organization of System Settings.
here are the shortcomings i've noticed:
* icon size is no longer configurable in System Settings. the dropdowns are still there, but they are not populated. as a result, i cannot scale up icons to look reasonable on my 220 DPI display.
* font size is no longer configurable in increments of 0.1 except when applying the same size to all fonts.
* adding fonts through System Settings is buggy.
* with Breeze Dark, some apps still end up with the same breakage that's typical of dark themes (text fields with extremely low contrast). it seems to be less breakage than with dark themes in KDE 4, but it's still there.
For more details, please see: https://wiki.kubuntu.org/VividVervet/Beta1/Kubuntu or don't wait, go straight to: http://cdimage.ubuntu.com/kubuntu/releases/vivid/beta-1/