My organization just completed a move from Zimbra to a hosted version of Exchange. Our overworked, underpaid IT wizards, with the help of the good folks at Rackspace Hosting, were able to complete the move over the weekend and, so far, I've not had a single problem. I've been able to access Email from Windows XP, Mac OS, Linux and even from my iPhone.
I'm hoping that when a person misses a meeting that the reason was a surprise customer meeting rather than "Zimbra ate the meeting entry." I'm also hoping that when one of my colleagues doesn't respond to a message quickly it is because he/she is really busy not because "Zimbra didn't deliver the message."
While no piece of software is perfect, the combination of product feature issues, product quality issues and support issues meant that Zimbra brought me no joy. I'm happy to say "goodbye Zimbra."