Have you looked at the beta versions yet? The UI was overhauled over basically the last 1.5 years and the points mentioned in the blog post are the bits missing to release that as a stable version (IMAP IDLE mostly): https://github.com/k9mail/k-9/releases
I've been running the betas since more than a year without any problems whatsoever. (If you don't need push obviously)
I've been running the betas since more than a year without any problems whatsoever. (If you don't need push obviously)