I installed a default Debian with the default graphical environment in a Virtualbox instance (64 bits).
After upgrade to Testing:
in /etc/apt/sources.list changed "jessie" to "testing" and also added "contrib" and "nonfree" repositories, then apt-get update, then apt-get dist-upgrade.
The process finished after about 2 hours.
After reboot, the system no longer boots into a graphical environment. Instead, it boots into a kind of buggy text console: blinking and non responsive. After a few minutes, the blinking stops and it's possible to log in, but also in text mode.
Thanks.