I'm not sure if the Kindle app just hasn't been updated for the 6 Plus' resolution yet, but on mine the text looks extremely blurry, making it slightly annoying to read. I'll continue to use my iPad mini for reading until they can update the app, which I hope is soon. I really enjoy the size of the plus, and having read news articles on it I could definitely see myself using it as my main ebook reader.
It was updated for iOS 8 but not specifically for the new screen sizes.
For my part, I retired my Kindle for my iPad a long time ago, though I have to admit the new super deluxe Kindle they just announced (for $200!) looks really nice. And I don't find myself reading outside much. Maybe a week or two on the beach every year and for that there's always actual books