Loving postie - have had it working in production now a couple of months.
We use it in 2 ways - regular posts and for custom post types (document// by @itthinx).
Since I upgraded to 1.5.20 attachments get added to media library but are missing from the post. This is happening for all posts regardless of post type.
I grabbed 1.5.19 from my dev site and put it back into production and it all works again.
Thought you should know. Happy to try to help you troubleshoot this. Let me know if I can help.
Only improvement I would make to postie would be to recognise categories on custom post types, either ID or name. I don't know if that is a limitation of the custom post type implementation or of Postie, but that would be fantastic.