I was one of those people wondering why I would use this and I have sent code via Gmail without problems. Personally I generally prefer plain text email. But I could imagine trying this out. Nice to see some positive endorsements.
It makes sense to keep the entered markdown as the text/plain part. But if this looks ugly, then the plain text view won't be as pleasant.
PS.: I hate all companies that think their newsletter only needs text/html, and put a "Please read me with an HTML-compatible mail client" as text/plain.