I am supporting Exchange for a large outsourcing company.
~6-7 clients 40-50 servers ~50K users. Exmerge is by far the most widely used tool. 3 of our large clients are still on 5.5, some with single server small sites where we could sell them on the idea of migrating to a brand new 2007 organization with exported/imported mailboxes.
But wait! WE cannot! We do not have the tools!
Exchange Server 2007 is yet another example of the sloppiness of Microsoft; releasing a software two years before it is ready for production use.
Our message will be loud and clear: stay away from it for at least two years.
Even though this is definitely not the first time, I still feel disappointed beyond my ability to express it