The biggest problem we faced, migrating a couple of months ago, was the discovery that you cannot duplicate an email to a folder, you can only move the original. The means, for example, you can't have a shared folder for the submission of expenses whilst retaining the original receipt in your own folder.

It's also not possible to block a contact if you have previously communicated with them without first deleting the contact from your address book. Why FM can't just do that automatically (and indeed, why it is a requirement in the first place) I have no idea - but it is really annoying.

Lastly, as someone else said, calendar colours frequently conflict.

So overall, it works - but lacks polish. It's almost like they don't dog-food their own app.

On the plus side, the data import from google was very fast and worked flawlessly for mail. We ended up in some confusion over calendars which I don't know the finer details of, however.