On my migration from Office 2003 to Office 12 - Outlook went from slow to Stop.
After long calls with the IT Helpdesk the problem was resolved. My PST files were 1.8GB big, the recommended size is about 1 GB.
It took a good part of 4 hrs to chunk that up into smaller PST files before Outlook12 started working with a speed that warranted the usage of Outlook and producitivity in the same sentence. Summary - Large PST Files - Bad.