Its greatest strength is an unsurpassed support for dealing with MailingLists. The threading is very smart, and it actually understands the notion of a MailingList: the list-reply command addresses a reply to a list post to the list but not its sender, avoiding cumbersome reply-to-all fiddling, and will not be confused by ReplyToMunging some evil mailing list software does either. There are many commands to allow you to efficiently select a large number of mails to operate on. Just about everything is customizable – keybinds, screen layout and colourization (including text highlighting in the pager), even the headers of all mail you generate. You can also edit all headers on the fly and individually for each mail. All of this functionality can also be bound to keys, making it simple to have an arbitrary number of mail adresses, mail servers, display configuration and anything else you can think of available at the touch of a key. It has superb support for sending mail from the CommandLine, even with attached files.
If you like this kind of power and freedom, Mutt is your mailer of choice.
Naturally, you pay a price for so much goodness: the learning curve is steep, esp since the default configuration is just barely usable, forcing you to claw through much of the documentation in order to achieve a comfortable environment. Those who are willing to invest the effort however will find themselves pleasantly surprised. No other mailer enables you to deal with high mail volumes even nearly as efficiency.
Unsurprisingly, it's very fast and takes very little memory (unlike, say, MozillaMail). Being a console program (can be linked against either S-lang or Curses) it is naturally very usable even across a SSH connection on a dial-up. Unlike PINE, it is completely Free software, available under the GPL.