K-9 Mail is good, but it doesn't support PGP/MIME. Only inline PGP. Doesn't look like they will either as it's been on their todo list for several years now with no progress.
Whilst this is true it is not really that big a problem. You can open the attachment, copy to the clipboard and then use APG to decrypt. It could be nicer and would be excellent if the client supported PGP/MIME, but I can live with it.
I would say it's a not-insurmountable problem, but definitely a big problem. If I have to copy-and-paste all my e-mails into APG just to read them, that's a significant inconvenience.