I had one issue so far, where after Update installation and subsequent first reboot, Apps launched would 'Not Respond'. They're launched, but not 'user-accessible'. Context-clicking on the Dock Icon showed a greyed-out 'Application Not Responding' item in the menu. I had to Force-quit six Apps (Safari, Viewer, Mail, iTunes, iChat, iCal) before I decided to unplug one USB device at a time to trouble-shoot it. I started with my Belkin Bluetooth Dongle, disconnected it, rebooted, and the reboot ('Happy Mac screen') took like 5 minutes! But, it did finally boot, and all is fine now. I reconnected my BT Belkin dongle, and all is fine still. I've read some little reports here and there where people have been having issues with some USB devices, drivers specifically.