For over a year now I have been running Cloudmark's SafteyBar product. I cannot recommend this highly enough. I get on average 100 messages a day. Of those 95 are spam. There are a large amount that are filtered out via anti-virus that aren't even in that count.
Today I installed Microsoft's Exchange Intelligent Message Filter add-on. It is a free add-on based on the same SmartScreen technology that is in the Outlook 2003 client. I decided to check it out after listening to some talks at TechEd that mentioned that this was all that Microsoft used as far as spam goes.
One of the cool features of it is that you can turn it on and let it just analyze incoming emails. I have done this so I can see what it is doing. Then as a further step you can see its rankings in Outlook as an additional column. I have turned this column on in my Inbox and Spam folder that the Safteybar uses. This way I can see how the rankings provided by IMF compare to Safteybar. After a few days I can then set the level on the server of what to block and what to just put in Junk E-Mail folder for Outlook 2003. I am interested to see how it works. Safteybar has about 95% hit ratio and I long ago gave up on searching for false positives, just to much junk to go through.
The additional benefit of the IMF is that you can stop mail at the SMTP gateway. This way the rest of your infrastructure doesn't even have to waste time and space analyzing the most obvious of junk mail.