In my experience, ditto.
Mail.app started off okay for me because some filtering was better than none and, as the article mentions, it's really easy to set up. However, good enough soon isn't.
I got SpamSieve a few months ago and after a single day of training it was catching much more spam than Mail.app. On my work account, which gets far fewer emails than my home account, SpamSieve not only caught all the spam but it didn't generate any of the false positives that Mail.app was always doing (something about email from Slashdot really rubbed Mail.app the wrong way no matter how many times I marked them as not junk).