- re-create meetings
- keep recurrence patterns
- preserve guest responses.
- book conference rooms and resource
- make it like your users have been using Outlook and Exchange all along.
- Everyone else who claims to do calendar migrations skips over all this functionality. To be fair, it is hard to execute correctly (and most of them have their hands full convincing you to do email when imapsync does it so well and so much less expensively), and it's not too important if you have a few hundred users. But it is the whole ball of wax if you're any real-sized corporation.
Zimbra to Office 365: