The next morning (9:22, according to SpamSieve’s statistics) I installed the free trial and gave it a whirl. By the next day, I was sold. There are 150 Spam messages per day that I don’t have to see. The accuracy of its filtering is over 99%.
I’m told Apple will put Bayesian filtering in their next release of Mail. For me, I couldn’t wait after seeing what SpamSieve can do.