We bought Zimbra NE to get the ZCO.
Unfortunately, it is quite buggy.
Issues encountered so far, after just a few users migrated from Domino to Zimbra :
- Location list remains empty when creating appointments
- Incorrect dates in message previews (but correct in message lists)
- Tasks not showing up
- no autocomplete from GAL (outlook limitation, not ZCO bug)

Zimbra desktop, on the contrary, works fine.
None of the above bugs encountered.

I hope VMware (Zimbra) will come up with a new version of ZCO very soon or I might be in trouble since I'm responsible for the choice of Zimbra over MS Exchange.