We also upgraded to Exchange 5.5 in 1999, and have not spent a cent except for a single support call @ $250, but with guaranteed resolution.
And we are not thrilled with Exchange as a solution, but annual expenses are currently $0. It will take some serious scrutinizing to go from that point to $1400/year (50 users). And what happens if we want to add users? How will single licenses be added? On occasion, we have added users from our parent company in Germany for collaboration reasons. Will we have to wait for updated certificates or some other similar vehicle to add users above the initial subscription?
How about the future? Will Zimbra remain a self-owned entity? We were burned by the Novell-Suse buyout. Shortly after purchasing what looked like a robust open source solution (OpenExchange 4.1), as we waited for anticipated enhancements, Novell decided that it just wasn't worth the effort/ competed with Groupwise or some other BS that made it even difficult to get the most basic support. Getting a login to their support site took weeks.
Regardless of these uncertainties, we are very excited about Zimbra, but somewhat hessitant to jump in head first. We are still waitng to see promised functionality and the Outlook connector. And hoping for some reworking of the licensing structure (perhaps a discounted renewal fee after the first year?)
Best of luck, and keep up the great work,
Jon