Ann Kirschner has a thought-provoking piece in the Chronicle describing how she read a 1,000 page novel across four different formats; paperback, audiobook, iPhone and Kindle. You might be surprised at what she decided was the best format, at least for her. As an aside, I'm now reading my fourth book in the Stanza iPhone application. I love the fact that it's always with me, and I can change the font and lighting depending on my situation.