My girlfriend has a kindle 2, and I have an iphone. I've found that the iphone has one killer feature that the kindle doesn't: It's in my pocket all the time.

For whatever reason, my reading happens in 15 minute spurts while waiting for someone or something. The kindle app for the iphone allows me to do that, along with all the book options that amazon provides (and synced up to my girlfriend's kindle too, so we share books).