The problem with GPG is the chicken-and-egg situation, along with user apathy. I literally don't know anyone I regularly email who uses it, so I can't use it either. Then there's the fact that virtually no email client supports it by default. If email providers like google actively supported and promoted it and provided public key management in the contact list along with a browser plugin for the private key management and actual encryption/decryption, we might even stand a chance of it happening, but google of all people will never directly support it as it'll prevent their full text search from working. (I'm aware of FireGPG - changes to GMail typically break it, so you can't rely on it)

I digitally signed all my outgoing email for a while, which did get some people asking why they couldn't read the attachment, (the signature is an ascii file but fairly useless to a human reader) but explaining email encryption and signing didn't cause any uptake ("nobody is interested in my email", "I have nothing to hide") so I gave up. I do still digitally sign important correspondance such as invoices and payment reminders.