I have to agree with this article. Since getting the Kindle app for my iPhone I haven't used the Kindle itself once.

There's no doubt that the Kindle is better for reading than the iPhone. But I always have the iPhone with me so it's easier when I have a few spare minutes to get some reading done.

And I often read in bed and the iPhone screen can be read without supplemental light whereas with the Kindle I need a light on.

The app has some useful features to reduce eye strain, too. You can change the font size, lock the auto-rotation, and change to white-on-black for night reading.