At this time, and somebody correct me if I am wrong, Entourage isn't a prime client target for Zimbra and has a number of issues:
- No native Mac MAPI/OWA support for shared data as in Exchange or Kerio.
- Entourage only syncs a calendar called "Entourage" with Sync Services, making Zimbra iSync connector very limited in utility. My early testing showed that syncing Entourage Sync Services Zimbra iSync works very poorly, i.e. problems getting the Entourage data back to the Zimbra server.
- Entourage categories are stripped of any meaning with a mixed sync scenario, when I managed to get it to work.
That said, Zimbra is a solid solution with Apple's native apps:
- With Entourage out of the equation, the Zimbra iSync Connector works well with iCal calendars and Address Book contact sync.
- The iSync connector also handles shared/subscribed calendars that users have accepted in the web client.
- CalDav with OS X 10.5 and iCal 3 is shaping up very well in Zimbra 5, including the ability to see free/busy. I have only tested this against the hosted RC2 demo.
Couple this to the fact that the Web client and Zimbra's forthcoming Desktop client are far more useful even than Exchange OWA running on Windows IE 7 (which lauds features Microsoft conveniently withholds from non-IE users) and you have a solution that we deem as a contender.
Since we have a company that is roughly 50% Macs, and have tried to keep our MS licensing overhead as low as possible, we have decided that the price and flexibility of Zimbra edged out our other consideration which was Exchange, and is relegating our CommuniGate Pro server to retirement.