I want to second that. I switched from PGP/MIME to inline PGP a while ago, since multiple of my contacts complained about troubles reading mails.

K-9 on android does not have support for PGP/MIME, probably wont have it for the time to come as it requires fundamental changes in the way the program stores the mails. There doesn't seem to be any open-source mail app on android that can handle PGP/MIME (correct me if I am wrong, I really like to know)

Windows Mail displays the mail's content as an attached text file, which is quite inconvenient.

On the plus side, several web clients I have encountered are now able to handle it. So there is hope.