The Debian packages for the 3.0 release work great. The weak spot is the zimbra-mta package. I have had to do a lot of manual tweaking to get Zimbra's MTA to play nice with other programs on the server that need to run /usr/sbin/sendmail directly.
I think an important enhancement for the Debian package for zimbra-mta would be to add a Relpaces: exim4-base and Provides: mail-transport-agent in the debian/control file. Then have a postinst script add a symlink from /usr/sbin/sendmail to /opt/zimbra/postfix/sbin/sendmail and all the programs that call sendmail directly would never know the difference. The added bennefit would be it wouldn't break APT and could be added to a local repository for installation/upgrade.
I downloaded the source tarball from sf.net and couldn't find any Debian build information. If you could provide a source .dsc some packaging issues could be expedited.