I've been using mutt exlusively for many years, and I got a fair amount of flack for it every now and then. While I like to believe that it's more secure than any full-featured bloated mail-client (hello Thunderbird), my reason for using mutt is simpler. I like the speed, efficiency and customization aspects of mutts, plus being able to use vim to compose my mails. Quite frankly, I don't get how people rave about text editors but then fail to include them in their mail workflow, which is where people spend quite a bit of time nowadays.

The other reason I like mutt is because it still conforms to the UNIX principle: One thing and one thing well. True, you could use mutt for SMTP/IMAP, but there are better tools for that. I use mutt with offlineimap, msmtp and mu (for searching).