I used Eudora Pro for many years, but I quit with OS X because I really hate the way its window handling works. I was using Entourage for a while but with Jaguar I switched to Mail.app.

Eudora is still the best thing for archiving email, since it can handle a huge amount of mail without slowing down and since it uses plain text files the database never gets corrupted. It can also search faster than anything else.