DRDUL I just switched to a Mac from Windows, and from Outlook to Apple Mail (via Outlook2Mac). Being the paranoid tinfoil-hat-wearing person that I am, I keep an archive of three years worth of mail. VacuumMail saved my bacon, as Apple Mail was choking on the huge number of message I imported and then started moving around. I use it regularly (it also runs automatically) to keep Mail "fit and trim." Highly recommended!
(Version 0.9.6)