Did an upgrade instead of a clean install too an hour ago.
However it took like a minute to boot up.
The white screen with the grey square on the left side of screen is just the sleep mode of the monitor.
I decided to restore to Lion again with a TM backup.
I probably will receive my Samsung SSD tomorrow and then will do a clean install on it.
Installed ML on my MBP yesterday and everything works but LittleSnitch isn't showing the live monitoring anymore in my task bar.
Imho, I don't find this upgrade that spectacular since I was already using iMessages beta, Used Growl and don't play games from Game Center.
I use Wunderkit & NotifyMe instead of Reminders, use iA Writer instead of Notes so what else important did change in ML?