SpamAssassin 2.50 was just released today. For anyone who has been using SpamAssassin but has been increasingly disappointed with its performance, this should make a big difference.
It incorporates Bayesian learning, so it can become highly customised to your particular email, and it even has an "auto learn" mode so you don't have to keep training it (but you can if you want to). And it has a much improved scoreset due to genetic algorithm improvements.
All in all it should be one of the better releases (modulo bugs of course