We recently migrated from Microsoft Exchange 2003 to Zimbra 4.5 Open Source Edition. The migration went very well and the documentation was excellent! Zimbra is very powerful and very user friendly. Our biggest reason for switching over to Zimbra is the flexibility Zimbra gives to our end users. The bulk of our users prefer to access there email via the world wide web. Most of our users complained how slow Microsoft OWA services run and complemented how fast Zimbra loaded and how fast you can do things inside of Zimbra.
Again very nice work Zimbra team and we also will start spreading the word about Zimbra. Thanks again!