My reason for considering Zimbra is two fold - only Evolution mail clients seem to be able to read/write successfully to our Ldap server using evolution's schema.... I need other clients to be able to use this application.... and the other is a lack of support for group calendars/sharing. Getting that setup and working seems to be a common problem unless your using MS products or GroupWise stuff.
I am hopeful that Zimbra can deliver the same performance we are used to with separate applications (Postfix, SA, Clamav) and is the right solution for our clients - calendaring and central directory services .
Please share your experiences with both and any other positive surprises you discovered.