I'm nearly a week into my Zimbra conversion and I am very, very happy. I'm still having some issues with Zimbra Desktop (sometimes it syncs (email), and sometimes it doesn't - this may be because I've yet to do a full sync because I've been on a narrowband connection while on holiday this week), but the web client is amazing. I used to say I'd never use it. Now I'm having a hard time figuring out why I need to use fat-client email at all.
The thing that has me happiest, though, is CalDav. John Robb, vice president of Marketing for Zimbra, walked me through set-up on Wednesday afternoon as I was leaving OSBC. Since then, my Zimbra-to-iCal (or the reverse) sync has been flawless. I change an appointment in iCal and it magically changes in Zimbra. No manual sync required. It just happens.
Now I need Zimbra Contacts to Apple's Address Book to work that cleanly. It's not a bad solution, but CalDav is soooo much better.