I have been using the Kindle App on my iPhone. I am currently reading Einstein His Life and Universe by Walter Isaacson. I am about halfway through it and so far, I really like reading it on the iPhone. Aside from not having the periodicals that are available on the Kindle, I have no problem with the iPhione App. The only drawback is that you have to use a web browser, either on your mac, or the one on the phone to actually buy books. You cannot buy books from within the app itself. While this is not ideal, I didn't find it to be a big deal.
Steve