I've been using PGP for a few years, and on the odd occasion, I'll send an encrypted email to myself. Part of the problem is that no one knows how to use PHP. I've been sending email to thousands of people in an academic setting, and I've only encountered one other person using PGP.
The reason I keep using PGP, however, is because of digital signing: there's a good guarantee that signed messages were actually sent by me. Headers are fairly trivial to spoof. With PGP, a 'hacker' can only impersonate me if they have access to the private key, which requires physical or ssh access, and he or she must be able to decrypt that key.
That said, I wish more people would encrypt their messages. This should be a no-brainer in a lot of fields, including human rights and for health and human services, and I think the barrier to commit to email encryption is still too great.