I have an iMac 9,1 (24/3.06/Rad4850/SSD) and I got hit with the silent reboot. I left the room when it said "9 minutes remaining" and came back 5 minutes later only to find the machine had rebooted. I just reset and booted into the installer and ran the Post-install and had zero problems. No PRAM intervention required. All is running smoothly, save for the cursor acting sometimes like it's a been shaken (Logitech mouse with SteerMouse Beta might be causing that). The CPU is also getting pegged as photoanalysisd slowly crawls through 75,000 photos, but that'll resolve in a few days.
So far it's running smoothly, and dare I say faster than El Capitan.