Looks interesting, but....
After having used Cyrus for many, many years in a production environment with overall great success, I have to admit at being very anxious with the idea of dumping that (and migrating all the users) over to what is in all honesty is an untried implementation. We also make extensive use of shared ("public") folders and complex folder ACLs. I have a hard time seeing it being practical to dump that, even for what appears to be a rather nice client. (I'm at least impressed to see that apparently Zimbra supports Sieve. Though, would that only be one script per user?)
I'm assuming the email messages are stored in MySQL, right? Do folks have any idea of the scalability for such an arrangement?
What about the recovery of messages/folders? One thing I've always liked about Cyrus is that headers are cached, but the messages remain in single files. While this might be simplistic, it sure makes recovery of messages/folders immensely trivial. How does one go about recovering messages when they're in a db?
One File Per Message Has Been Tried
Visual SourceSafe, the Microsoft source control tool, has used, still uses, one file per file version, which is equivalent to one file per message as Zimbra is using. With low total numbers it works OK; it does not scale. I see no reason to expect a different result here, which is unfortunate, because with the addition of a few other tools common to Notes or Groove, this is a dynamite product.