I migrated our MobileMe accounts to iCloud the day it was released, after updating my iPhone to iOS 5. I was using two accounts. One account is my primary and I synched mail, calendars, contacts, etc using it. I had another account that I only used for e-mail.
Overall, my transition to iOS 5 and migration of the accounts went without any major issues.
Well, that was the case until I tried to use Pages, Numbers, and Keynote on the iPhone with the new iCloud sync capability. For each app I turned on the “Use iCloud” option but every time I opened one of the apps the setting reverted back to off and in the iCloud Website it just showed the intro pages for the apps.