Being an old windows user: CLEAN!
Being an 1.5 year OS-X user: Probably clean, though having doubts. Guess I'll go for Archive & Install what looks to be quite nice 'between' feature.
But still... Think of all the junk you still have from old de-installed software (yes, you've got those files too in OS-X). Or all those program's you don't use anymore etc.
What's better than starting a new OS on a new fresh empty disk, right? 😀
But mmm... i'll wait a day or 2 before installing and i'll keep a close look at the fora for yet unknown bugs (That blue-screen thing with upgrading looks like the first one was maybe found, quite normal if you ask me [Windows XP was a pain in the ass too the first day with me, as was Vista, as was bootcamp 1.0, etc.etc.etc.].