imapsync is more than capable of handling your Email migration. Buy the software and support.
Cards on the table here: we do not get any money from imapsync, but we've found it to be a quality product with a great guy behind it and a good support community. Add to that it's inexpensive nature and liberal software license and you really cannot go wrong.
You will need to make sure you have permissions on the Office 365 side if you wan to do multiple mailboxes without individual passwords. See our earlier posting on using imapsync to migrate email into Exchange.
You can also handle migration problems with the wealth of Exchange information in their FAQ:
Other very useful FAQ solutions include what to do when encountering individual attachments over the default limit set by Exchange (10 Mb) and Office 365 (25 Mb), as well as line length issues.
We're working on a DIY document to take you through your own migration. Frankly, we get lots of questions that could be handled by such a guide. And we're tired of Microsoft black boxes and over-priced third party offerings.
We'll offer a "flat" calendar migration in an imapsync-like model just so folks can get their migration done if they don't mind a simple calendar migration. Contacts and tasks are included in this of course.
The goal here is to get your software costs for a migration to Office 365 to under US$1000 with you supplying your own elbow grease.
In rough numbers email, calendars, tasks, and contacts should be about 95% of the data you need to move.