I downloaded ML, put it on a flash drive, backup up a few files to external hard drive, re-formatted my laptop, clean installed ML from the flash drive, entered Apple ID, entered e-mail settings, re-installed a few apps from the app store, copied back over the files from the hard drive and was done.
And, ML works better than good on my late '08 MacBook Pro.
Seems great to me. Things move forward...can't stay on old OS's forever.
I'd venture to say most people don't have issues with ML and it's installation process,even when clean installing. Doing the upgrade over an older version over OSX is even easier than what I did.