Last Friday (July 25th) was the official day I lead Menlo Park City School District in moving mail services from a plain ol' OS X mail server to Zimbra open source edition. Let me tell you, with a lot of planning the migration went very smooth. Any problems we encountered (which were minor) were due to user error.
I have to give my thanks to the extensive documentation and community support that is available for Zimbra. The migration wouldn't have gone as smooth as it did if it wasn't for t the forums and wiki.
Zimbra is awesome!