Very recently (2003-08-11) I started using spambayes[6] in place of spamassassin. spambayes uses an entirely different approach to identifying spam than spamassassin uses, although I have recently been informed that spamassassin incorporates the same bayesian filter technique[7].
spambayes is working well for me, even though it doesn't provide server-side scanning and SMTP-time rejection. YMMV!1
(Naturally, if you are trying to implement server-side content filtering for a several (or more) users then spambayes is not suitable for your deployment and you should definitely use spamassassin instead.)