I first began to use K9 when gmail's android app stopped allowing me to send plaintext email. I still find this very confusing. I routinely write emails to a developer using an ereader, for whom plaintext emails are trivially easy to work with and for whom HTML emails are a world of pain.
That's what started my migration away from google.
Having said that, K9 is fine. It's good enough that I'm not looking for a replacement.