Stanza has redefined how everyone thinks about reading on a mobile device. While the iPhone is not the best device for reading electronic books, it's the device that we have. And Stanza does a great job at making the most of it and even in a free app.
I hadn't realized Stanza was free. Well, no wonder it's popular! So if you've got an iPhone, or an iPod Touch, you've got an e-reader with no extra expense. Except, of course, for the ebooks, assuming you buy them instead of just grazing the web for free ebooks.
Still happy with my Kindle.