Cool. I'm a FreeBSD user for a long time, have always preferred ZFS even on single disk systems because of Boot Environments.
Doing a major upgrade I've always done the "create a new BE, mount it, chroot to it and do the upgrade" process.
That lets you do the upgrade kernel, upgrade userlande, upgrade all the packages into the new BE while you are still running, then when you boot into that newly created BE everything is consistent, so you have a lot less risk of things going bad. They still can, but that's where the bootonce flag comes in. If the system fails to boot up completely (where the flag gets cleared) reboot and you are back to pre upgrade.
Automating this process is a very good thing to have. Very good stuff Netgate.