Using it at home.
I admit I set up a PC using parts I had lying around and installed Zimbra (open source) on it. And it works pretty well - integrates with Thunderbird as an IMAP server, has its own desktop client, and does the web-mail thing rather nicely using AJAx and HTML (and even has a cut-down version for mobiles).
There's a few problems I'd like to see fixed with the HTML version, and there could be better integration with external mail servers (it is there, but only in the AJAX version), but on the whole, it works well as a mail server (and the SPAM/HAM filtering works well).
HOWEVER. One thing I really wanted was the ability to sync my contacts/calendar with my phone - and this functionality is only available in the Enterprise edition. I'm sorry, but I won't pay enterprise prices for what I essentially use as a home mail server. (as for FUNAMBOL, it installs and runs but the Zimbra plug-in keeps on complaining about Java class version problems, even though I compiled both on the same PC)