JAZZYGUY  This is NOT an application that can simply be placed in your applications folder and then launch it and configure it for your pop mail account in mail.app First you have to install the developer tools and then use Terminal to go through all kinds of permutations and commands. This app is for the users who know how to use Unix and are one of the brave and fearless. Unfortunately I am not one of them although I have dabbled with Fink and X11 I am depite all my years of experience with my Macs an ingenue and dilittente nevertheless. More power to those who can use this!  
(Version 3.2.1)

FOULGER  Great Product - worthy of a UB upgrade. Thanks to the developer for providing this essential tool free of charge.  
(Version 3.2)

BRIAN KENDIG  SpamAssassin is superb, especially when paired up with a mail server that makes good use of it. I have a Mac OS X 10.3.9 machine (not Server) on which I run the Exim mail server software, hooked up to SpamAssassin via SA-Exim. It's all free software. The mail server uses SpamAssassin to identify spam before it's accepted, so that it can refuse delivery. The result is that the spam never even appears in my junk mail folder; it's refused before it even gets to me.  
(Version 3.0.3)

ANONYMOUS  SpamAssassin is not magic. If your ISP/web hosting provider offers it and it isn't working well, they haven't set it up properly. I get hundreds to thousands of spam message per day on one account, and SpamAssassin is by far the best tool for filtering it out.  
(Version 3.0.2)

VISH VISHVANATH  SpamAssassin, when configured correctly, is an excellent package. I run it on my FreeBSD 4.9 server, as a module activated by MIMEDefang. SpamAssassin plays beautifully with Razor and MIMEDefang and returns perfect hits on all incoming spam. Incoming viruses are discarded before reaching the mailbox and spam tagging is triggered at 5.0 points. The combination is *so* effective, it has tagged 100% (Yes, 100%) of all spam in the last three weeks. And 0% of all legitimate email. Impressive. I am considering discarding all tagged email, so confident am I becoming in this combination. In any case, Apple Mail picks up the X-Spam-Score header and deals with junk mail appropriately.  
(Version 3.0)

CONSTANTIN  SpamAssassin is a godesend. Here is an intelligent way to tag spam at the server level, then decide what to do with it. Using procmail or whatever, spam-tagged mail can be deleted, for example. More importantly however, is that spam can be stopped at the server level, i.e. before tying up your internet connection, computer, etc. (particularly if you're using a dialup connection).

The new approach of blacklisting spam according what web-sites it is pointing to is brilliant, as it makes it that much harder for internet scum to succeed overcoming spam filters.  
(Version 3.0)

SPAMHEID  pretty rubbish. spam assasin is installed on my webspace providers servers and seems to make no difference to the deluge of spam i receive daily [even when the subject lines and content are blatantly spam] - however it does seem to reject perfectly legitimate emails with annoying regularity  
(Version 2.6.4)

JOEL MUELLER  SpamAssassin is great, even if you don't choose to automatically delete emails tagged as spam, because it can add a subject prefix to the incoming email like "{spam}". Then you can set a rule in your email client application to auto-move those to a specific folder for review.

I only wish that SpamAssassin would be built to support client-side configuration based on a single domain so that shared hosting companies could more easily allow their clients to configure it.  
(Version 2.6.1)