Since purchasing a new iPhone 6+, I've retired my 2011-model Kindle reading device, and now do all of my Kindle reading on my phone. (It's a lot easier than carrying two devices around.)
Yesterday my iOS reading app updated with the new Bookerly font described in greater detail in the above article.
Overall, I see a significant improvement. I recommend updating your app if you read Kindle books on your smart phone.
As a company, Amazon makes mistakes, to be sure; but the company usually does things right. If only our government were half as efficient and innovative as Amazon.