We are now using S/mime and in today's Apple+Thunderbird products it's completely built-in and pain-free. Set it up once and after that all emails get encrypted automatically, you don't even need to press a button. Provided of course that You were able to convince your colleagues to invest those 10 minutes to set it up as well.
PGP Was painful because every Mail.app update broke it, not sure about the current state-of-the-art there. But the whole point should be: it is not much of an annoyance anymore! Zero annoyance after installation, Works even on your iPhone etc.
thinking that the Chinese government might have built in a back door gives me a little pause though