I've been running beta versions of Outlook 2007 at work for a while now, and I just love it.  They've got a bit of work to do on stability, but that is to be expected for all pre-RTM software.  Why should you upgrade, you ask?  What makes Office 2007 different than all the other versions?  Well, there's the standard package of bug fixes, additional features and slick UI, but the two main reasons, in my opinion are the new Button Bar and the productivity features that integrate Office with Sharepoint and the web.

Here's a Channel 9 video of Outlook 2007 (with my favorite feature: RSS integration)

"Jessica Arnold from the Office team takes Scoble on a quick tour of the new features of Outlook 2007 including the new "To-Do Bar" that integrates your calendar and tasks into one quick easy view, in-place attachment viewing, and more."

http://channel9.msdn.com/showpost.aspx?postid=195021