I use it wherever I can, but encryption is something that requires the participation of all communicants. The only option for an unwilling or unable participant is to be excluded from the mail completely and contacted in a separate channel (preferably Real Life). This, combined with the fact that encryption requires effort, makes it very, very hard to propagate.

Encryption for mail will remain practically zero until someone implements an extremely simple, integrated user experience. It's one of those things no normal person will acquire unless you ship it with Windows and make it totally transparent.