I have now tried the Exchange Server Intelligent Message Filter on our Exchange 2003 Server for a few weeks. The amount of spam has dramatically been reduced and I really recommend all Ex2K3 adminstrators to install it. You can download it for free here.
The current configuration we use is to classify all mail with a SCL value of 6 or greater as spam, and that level is really great. A few spam mails is slipping through but no mail has been lost (we currently archive all mails that are classified as spam, and checks them each day).
Read more about the story behind IMF here, the same technology (SmartScreen by Microsoft Research) is used in Microsoft Outlook 2003, MSN 8 and Hotmail for detecting spam.