We are currently testing the open source edition of ZCS, and to be honest with you, we are fascinated with the product. The email server has been up and running for 4 days.
We currently have about 250 email users in our institution, and we have been running Microsoft Exchange 2000 and currently migrated to Exchange 2003. But as with several Microsoft products, we had to troubleshoot the mail server every couple of months until we just decided to try something under Linux.
Do you have any other Licensing (for the Network version) scheme that allows Exchange users to migrate without making such a high investment, (500 minimum users x $28 = $14,000)? Our original plan was to migrate to Oracle's collaboration suite, wich is only $6 per user, but after trying Zimbra, we might decide to stick to the product if we can get a better price than what Oracle offers.