I decided to try out the Fedora 20 custom spin with the bleeding-edge "KDE5" development packages to see how it's working. In the few minutes I spent toying around with these very latest packages, I was impressed. The desktop seemed faster than with KDE4 -- likely due to being able to better utilize GPU acceleration and other optimizations with Qt5 -- and overall it seemed to be a nice upgrade. However, it was not without its share of bugs and other shortcomings, but for still being more than one month away from the release, things are looking good; KDE Frameworks 5 and Plasma-Next are scheduled to see their official releases in July. KDE Frameworks 5 should be final on 1 July while the Plasma-Next release is expected on 15 July.
Among other options for trying out the next-generation KDE stack to KDE4 is via the Arch Linux packages, an F20/Rawhide repository, and Project Neon / Kubuntu Experimental.