Successfully upgraded Win7 Pro to Win8 Pro at midnight.
The whole process (paying [via paypal or credit card], downloading, creating .iso, then upgrade) took, all in all, 45 minutes.
*I have a fast internet connection and i7 with ssd so that helped making things fast but regardless of that, it went really fast (compared from Vista to 7 upgrade).
I'm still going to perform a clean install tomorrow, though. I want to see it / explore it / tweak it from it's default settings instead of from an upgrade wiith my settings saved (search disabled, indexing disabled, hibernation disabled etc).