Seems to have converted fairly seamlessly - I got an alert in iCal that it was "upgrading" my calendars, which never went away - but after checking all the calendars were correct on my phone and in the cloud, I quit and relaunched iCal and they're fine.

If I understand it correctly, using it only for the mail/calendar/contacts and the odd photo that I was using mobileme for, the icloud service will be free where I had to pay for mobileme (unless I want more storage above 5Gb). Is that right?
If it's brokenless, don't suffix it...