FYI, Apple will incorporate spam filtering in Mail.app as of the next release. It's already been shown on their website.
I currently use SpamBouncer, it's the same sort of ranking system as SpamAssassin, so it's not like this is system really anything new. In fact, SpamAssassin looks like a huge pain in the ass to set up compared to Spambouncer. SB uses Procmail, SA uses Perl. SB is a lot easier to set up.

Remember to update your Developer Tools to the latest release before you try to install SpamAssassin. I tried to do it before updating them, and the perl modules required wouldn't install. Afterwards, though, it installed like a charm from Ben's instructions.

The awesome thing about OSx is that Apple could ship it preconfigured with whatever groovy *nix apps it wanted, including procmail.

SpamAssassin has been an excellent product for me and many others. Am really looking forward to the Outlook and Exchange versions that Gordon and Craig are producing soon at Deersoft (http://www.Deersoft.com). Installation should be point and click. for the web download version and the corporate server version. Maybe this will make it easier in the Windows world, Sakusha.