A note for those of you using nVidia's web drivers: this update makes the system incompatible AGAIN, so it falls back to the base OS X drivers. (Way to go, nVidia.) If that annoys you like it does me, you might want to hold off on this update until nVidia gets a clue.
This update addresses the problem with a series of keys that could affect 2012 Mac systems. This update is recommended for all Mac systems that have been introduced. Markets in 2012. Only.
I have a problem with keychain With MacBook Pro 2012
If you have a recovery partition set to an external drive, make sure you verify the disk which will update the "boot support partitions".
My RAID configuration doesn't support the recovery partition so I had no choice but to set it externally. If you set it to another drive for any reason, just verify it so you won't be stuck when you really need it.
This update failed on my 2011 15" Macbook Pro with OWC SSD. After several hours "hung", I had to force a restart, after which my SSD boot drive was completely hosed - not even visible.
This is the fourth time I have had to RMA one of these drives. Not sure whether this was caused by the update or not, but I would avoid this update for now.