Ray, as a digression I can report the use of the RBL spamhaus.org "combo address" resulted in a 75% reduction of SPAM to my site. My domain has been live since 1993 so it is reasonably well known as an address to be targeted

My 6.01 server is a server only as I use a hardware firewall.
My only problem with SpamAssassin, which I do use, has been the amount of tweaking necessary to really make it work properly. The GUI is a stab at making it easy but overall the "learning" abilities are not optimized, neither are the settings for thresholds. In order to really make it work efficiently there need to be some "signatures" included as well as a much more granular approach to what is tagged as SPAM and what is deleted, which in turn allows the Bayes filters to learn to a higher degree.
I think there is a misunderstanding that the Bayes filters "learn" in some exotic, mystical way. They do not, but they respond incredibly well to specific tuning.
Out of the box, the excellent "SA for dummies" contrib only gets things to a point, then the SPAM remains at a constant level. Editing .procmail results in much more finely tuned learning. The Bayes filters are really capable of granularity. Unfortunately, they also require more tuning than an out-of-the-box contrib- as herioc as it is! I am not criticizing the SA contrib in any way, just saying if some of you are not overjoyed, it is due to the implementation not being complete.
I would say, in general, the use of mailfront contrib in conjunction with something like Thunderbird's built-in Bayes system would cover over 95% of the SPAM quite easily. Thus, to offload the CPU on the server, it may well be a better idea to not use SA unless you invest the time to learn about it and really fine tune it.
Digression: the most powerful use of SpamAssassin, in addition to other really useful features, can be found at
http://www.barracuda.com. I've never seen a box do a better job at blocking SPAM, ever. I have no relationship with these people at all other than installing one of their boxes at work and becoming a hero to the user base.
regards,
patrick