I primarily use Kindle on my iPhone 6+. I had a iPhone 5, and when the newer iPhones came out last year I thought the huge thingies were just going to be too big to carry around. Then a few months later a friend let me play around with his iPhone 6+ and my attitude changed in a second! I got a 6+ shortly afterwards. I really love the large screen and it has become my primary reading device. I still like the iPad Mini Retina for reading, but rarely use it since the iPhone 6+ is always nearby--usually in my belt holster.

As others mentioned, iBook and Kindle apps still have big margins, but that is not a deal killer for me. I got tired of deDRMing every damn book and adding custom fonts and what not. Why spend more time modifying than reading? With the retina screen and better fonts I no longer need to do all the modification prior to reading, and using the Kindle app with their DRMed files means I get the full syncing capability that is a PITA with our own or modified books. I simply got tired of fussing over the minute details of formatting and just started reading. The technology now allows me to that.