I have recently updated (3 days ago) my Debian testing Plasma 4.14.2 to Plasma 5 successfully, but it was not trivial.
First, I tried to do the Plasma 5 upgrade by migrating from stable to testing and performing the typicall apt-get upgrade && apt-get dist-upgrade. However, such operations intends to uninstall packages such as "task-kde-desktop", "kwin", "kde-standard" and so on. I accepted and obviously the DE brokes after the reboot. I tried to perform the upgrade by other similar ways (using aptitude instead of apt, etc) and always brokes.
So finally I decided to perform a fresh install of Debian 8.2 WITHOUT any desktop environment, then install the KDE Desktop through the command line and it works perfectly!
The steps I followed were:
1. Fresh install of Debian 8.2 (stable) WITHOUT Desktop Environment