In E-Mailer' s case, a set of buttons are added to the top of each message window. They allow you to lookup someone else's key, encrypt, decrypt, and authenticate messages. The integration just isn't very smooth. Whenever you send a message, you must click the encrypt button and enter your PGP passphrase. This security measure prevents someone else from impersonating you by using your computer to send out e-mail, but it becomes a nuisance if you send more than five or six messages per day.
Many people send ten times that.
Another problem is that if you use PGP to encode mail you send, it remains encoded in your sent mail folder. More importantly, I think it could inspire confidence in the Internet and technology in general. I know I do. With all the e-mail clients out there, you'd think that someone could add a useful feature like good security integration. I hope they will.