I prefer the Kindle app on my iPad. You can't beat the convenience
Val-
I love having a book in hand, but kindle is making it hard to stay committed. Somethings are easier when the book is in front of me (referring back to things).
That said, for the same reasons as Val, I love Kindle on my iPhone. Plus, I also have it on my laptop so, I can pickup where I left off on either device. And of course, it travels well.
Also, my daughter has recently found this new app called "Spritz" which makes reading faster easy. This is perfect for people like us that have to review books. I don't love it for enjoying a good book, but for quick reading to review a book, it is perfect. You just load the kindle book into the spritz app. I increased my reading time by 45%.
Finally, I am starting to clear my home of excess items and most of what is left is books. So, I really need to make the switch where I can.