KDE's Plasma Next is written against QML with Qt5, Qt Quick 2, and an OpenGL / OpenGL ES scene-graph. Compared to the Plasma 2 alpha releases in months prior, there's been more stability, performance, and bug fixes that have landed for this upcoming KDE overhaul. This beta of Plasma Next is also shipping the new "Homerun Kicker" Application Menu widget and is shipping the Oxygen font too.
The initial stable release of KDE Plasma Next is expected in July, assuming all testing goes well. The KDE developers acknowledge there's still improvements to be made with regard to stability, the performance of Plasma Next in certain configurations, and more QML2 polishing to happen.
More details on the current state of Plasma Next can be found from today's beta announcement at KDE.org.