i've finally got this installed on my mail server so can use it in anger. it takes a while to get used to the very different interface and philosophy that zimbra takes, but its very impressive so far! a few observations:
1) the interface works beautifully on 1280x1024 desktop. however its very difficult to use on widescreen laptop @ 1280x768 (fairly common for road warriors using webmail). the vertical segments are too high and don't resize well, so the actual contents of the email at the bottom of the screen only get a few lines of reading space. a lot of this space is taken up by dark(er) gray subject box with tag optionally underneath, and the message header box. it would be nice to either eliminate or merge these boxes, or have them collapseable, ala thunderbird. also, in conversation view there's a limit to the amount you can drag the message window up at the expense of message fragments. hope this makes sense.
2) i cant see a way of seeing full message headers.
3) in conversation view, you can't resize most of the columns, and messages with a year in the received column wrap off the end.
4) in 'By Message' view, it would be nice to have a collapse/expand thread view - this is what a lot of people are used to and would make transition much more natural.
5) calendar doesn't remember ticked calendars (i think this may have been flagged as bug 4708, if memory serves?)
6) please please please please please add a todo list integrated with calendar ) most people work more productively with checklists - it makes projects and time management much easier, and would be a natural fit for zimbra.
this is groundbreaking software, thankyou for contributing it to the opensource community.