This is basically for my information, but maybe someone else will find in useful as well. From TechRepublic's Brian Posey, 10 reasons you might want to reconsider upgrading your Exchange setup. Read his article for the full skinny. Here's the short list:

1. 1: You may not be able to reuse all of your server hardware
2. 2: You can say goodbye to any legacy Exchange Servers
3. 3: You’ll have to make changes to your Active Directory
4. 4. You’ll need some retraining
5. 5: Front-end Exchange servers have been replaced
6. 6: Routing groups and administrative groups are gone
7. 7: You’ll need to plan for backward compatibility
8. 8: The Client Access server has a built-in proxy
9. 9: You’ll have to learn the server roles
10. 10: You’ll need to use PowerShell for some management tasks