While I personally did not find the spam protection to be sufficient, you might find the following of interest and useful if you are using or deciding on whether to use Mail as your email app. I am still running SpamAssassin at the server level and using SpamSieve locally to pick up things that get missed.
Many myths have emerged about Mail’s junk mail filter. No, it’s not an extremely complex set of rules, no it doesn’t look for keywords, and no, it doesn’t use white magic. To truly understand what makes it so much better than the competition, we’ll have to take a closer look at the recognition engine and the technologies it relies on to do its work. It may sound a bit complex at first, but things will begin to make sense as we work through the mechanics. [MacDevCenter.com]
I do like Mail.app a great deal and hope it gets better in Tiger this fall. Entourage has been probably been updated in the new Office and Thunderbird continues to grow as well.