Two thumbs down for Mac OS X 10.6.5
This morning, I installed the update for Mac OS X 10.6.5.
After the installer was done and my MacPro rebooted, my primary monitor no longer worked.
I have a KVM that allows me to share a pair of Dell monitors, one 19 inch and a 24 inch, between my MacPro and my HP Z800 workstation.
Once I installed the update, the 24 inch monitor was no longer detected by the operating system.
Thankfully, I use Time Machine and Apple prepared a document for such a situation: reverting to a previous Mac OS X version.
The scary thing abou the process though is that any file that was created after the full backup is lost and you need to restore that from Time Machine as well.