Upgraded a 4 node production cluster running Ceph with no downtime or issues. I did do the needful and ran the check script, ensured Prox 6.4 was fully updated, and reviewed my systems for any of the known issues/extra steps. For example, 3/4 nodes in this cluster run old boards and therefore can only BIOS boot, not EFI boot, so I made sure their ZFS boot pools were properly switched to the proxmox boot method. I have not yet upgraded Ceph to Pacific due to time constraints.
10/10 major version release. Lots of nice little UI improvements, some apparent performance improvements on my hardware, and continued support for hardware that should have been recycled years ago.