After trying it out for a month, I pulled the trigger and bought MailMate this morning over a cup of coffee. Gabe Weatherhead has a very good features overview (now a bit dated but still relevant) of this powerful mail client.
Good software is worth paying for. Even though mail on OS X isn’t my hub of productivity that it once was (being that my day job is on Windows and all), I’m convinced that a mail client as good as MailMate is worth the cost. If you try it out, don’t miss the custom keybindings and hidden preferences. You can do a lot with this sucker.