There have been a few hints on the site regarding using the CLI-based email client mutt, but they've typically been little snippets about how to accomplish certain things. Being an avid mutt user on Linux, and then on OS X, I've written an article called Using mutt on OS X, which is a full guide on using mutt, covering everything from mutt configuration (with sample configuration files), to setting up fetchmail and procmail, to even tweaking the postfix MTA (or installing and using exim instead).
It details a number of OS X-specific items, such as integration with Address Book, being able to automatically open attachments from mutt, etc. A few of these are from previous hints that ran, but this article takes you from having never used mutt all the way to being able to power-use it.
Using mutt gives you the most rich and flexible email client experience currently available on OS X. Yes, it's a commandline client, and as such doesn't have all the mouse clicks and sounds and whatnot that something like Mail.app would have, but it's stable, handles huge mailboxes, and is customizable like you would not believe.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070303204615295