"Usability" is ill-defined. GPG seems to have pretty good "usability" from mutt (which auto signs outgoing email and optionally encrypts with a keypress). Maybe your MUA is broken or you forgot to RTFM.
Usability (as I've employed it) is when someone non-technical can derive benefit from something beyond their understanding. Rest assured I've read every line of the GPG man page and then some.
