"Interesting. I've simply done an upgrade install on my various Macs ever since I've had Macs... since 1984. Never had a problem. Not even once. That method is Apple's recommended method for doing an OS upgrade and has been for, well, years, decades even.
Is Mickeysoft quality really that bad?"
One thing apple fan bois like to forget is that with a Mac, apple controls more or less the whole eco system of hardware and OS. With Windows, Microsoft have to cope with almost limitless combinations of system hardware, while migrating from one version to another does work, it's not always the slickest of systems going forward. It works, it's just not optimal.
James's Apple fan boi comments aside, a clean install of any OS gets rid of traces of naughty programmes that don't uninstall properly and generally give you that new system sparkle, every time there is an article on EL Reg about this they give the impression you have to reformat your drive to do it and it's a major issue. When in fact it's a minor inconvenience. In this case moving from a free beta or free RC, it’s not too much to ask to do a fresh install, let’s face it if you’re not in any way technical a pre release OS isn’t for you. Microsoft make this quite clear when you sign up for the beta product key.
I run the beta of Windows 7 and personally I’m looking forward to a nice fresh install of the RC that will fix the very few bugs I have come across with the beta.