Martin Gräßlin reported today that the next-generation KWin code is to a state where he tried replacing his currently running KWin with the experimental code using Qt5. The new code isn't yet fully functional but after fixing up some regressions, most of the key functionality is now present.
The KWin compositor Martin is presently testing with is the OpenGL 2.x back-end using EGL as the GLX version is currently broken. Martin's also hoping for EGL driver adoption ahead of KDE Frameworks 5 so that they could just drop their GLX code. The open-source Mesa drivers have EGL but the binary AMD and NVIDIA drivers are still what's holding up the EGL adoption process on the Linux desktop, but that should change soon.
More details on the latest KWin developments can be found via Martin's blog.
Those wanting to try out the experimental KDE Frameworks 5 related work can download Project Neon packages.