I imported public keys but could not use them in K9 to encrypt emails. Instead  the app insists on creating a new key pair or importing secret key also. What if secret is not available (as in most cases). I never had a problem like that with APG. I also find UI of APG superior. Unfortunately  it looks K9 dropped support for APG. The only good thing is that this app support some EC encryption  hopefully non-NIST curves too.
February 14 2017