For quite a few years I have urged nearly everyone I know who is even marginally computer literate to use PGP or OpenPGP to secure email, with exactly one success, who already was set for, and using, one of these product.
IT security is one of my fields; I've have GPG installed on all of my computers for years; I have a thorough understanding of cryptography and a passing familiarity with the specifics of the PGP, PEM, and S/MIME protocols. I don't bother encrypting or signing any of my email.
Why not? Few or none of the recipients are prepared to do anything with either, and the presence of signatures would only confuse them. And there's very little benefit to me in sending encrypted or signed email, even if my recipients did handle it correctly. My email just isn't that valuable (except internal work email, which never leaves the corporate network, so an attacker who gained access would almost certainly have stolen creds to read it anyway).
I think promoting secure email is a quixotic quest. Yes, if we could get most people using signed email, it'd at least cut back somewhat on phishing and the like. But the threshold for that to be useful is very high. Beyond that, it's mostly useful only if two parties agree beforehand that their threat model justifies it, and they configure it as a special arrangement. For everyone else, it's "oh, there's one of those weird paragraphs of garbage at the end of this message".
In my experience, the chief use of PGP/GPG is to sign software distributions, which has some utility, though many organizations don't practice any sort of consistency or provide decent key verification (hello, openssl.org).