Already have done
With the onset of "cloud based" services I think Zimbra's going to have to concentrate on these collaborative features in order to stay in the game.
I've been running Zimbra since the start of this year and my users always talks about Google Apps and Microsoft's online efforts.
I am running the 60 day trail of NE version 6 at the moment and the webDAV / document collaboration's quite poorly done. I think we're going to move up to 7, in which this is meant to be improved, in the very near future but the users are already decided on Dropbox for docs because Zimbra wasn't up to the task
I'd like to get them on to Zimbra.