I always kepp GnuPG loaded, with the GPGMail plugin for Apple Mail, and GPGKey for managing keys, it offers most of the commercial PGP functionality in an open-source solution.
The problem: none of my friends or colleagues would even consider encrypting their mail, even if it pertained to personal, or even financial, information. They consider it too much effort. These are the same people who use unpatched Windows XP systems and open any attachment from a stranger claiming in bad english to have a fix for teddy-bear...