Things I really like about it -- the way accepted, declined, and tentative meetings show up with varying degrees of transparency. Very intuitive, very elegant:
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There's also a fairly decent schema for resources.
On the other hand there's zero concept of private activities or degrees of delegate access.
Calendar migration from Outlook via PST is better than we typically see with most packages. Zimbra manages to preserve the recurrence pattern of meetings and exceptions, and there is an option to change your email address on meetings from your former email to your new one if you're changing.
On the downside, it doesn't change guests' email so when I brought it up in our test lab (changing domains) I had outdated guest lists. Migration is also done on a single user basis (making it a pain if you have a large base you're migrating).
I didn't bother with the Domino or GroupWise migrators.