Upgraded a few of my systems from 11.1-RELEASE-p11. So far so good.
Remember to rebuild VirtualBox kernel module after upgrade.
After upgrade from earlier version (11.1-RELEASE-* / 11.2-BETA* / 11.2-RC*) loading the VirtualBox kernel module will provide you an instant reboot. Disable loading that module at start (if you do) and rebuilt the /usr/ports/emulators/virtualbox-ose-kmod port.
From what I recall You will need to have FreeBSD sources at /usr/src to do that.