How is it to use? Actually, pretty darned good. Readability is high, and you can change the font size to your liking. Turning pages is just a matter of swiping your finger across the screen. A single touch gives you access to the table of contents of any book, and you can bookmark pages.
I liked the idea of the Kindle app so much, I actually broke down and bought the Kindle 2. The two devices interact, and you can have all your books on both devices. In fact, the iPhone Kindle app will come up on the same page you left off on the Kindle 2, or vice versa. I thought that would be an automatic function, but you have to tell each device to sync when you leave. Your mileage may vary, but that's what I am seeing.
Of course, even that rather cool function is a bit tricky with the iPhone, because its smaller screen can't display as many words as the Kindle 2 screen. Even though you are synced, you won't get a page that looks exactly the same. No big deal, though.
Read on for more... Illustrations look pretty good, and you get color on the iPhone screen, while the Kindle 2 remains grayscale. I'd like to be able to zoom in on those images though, which is something I can do on the Kindle 2.
What are the other downsides of the Kindle for iPhone app? I'd like to be able to read in landscape view, but the app only supports portrait. The dictionary feature is missing. On the 'real' Kindle you can move the cursor to a word and get a dictionary definition.
Finally, there are no magazine or newspapers subscriptions available on the iPhone side. Even if you have those subscriptions on the Kindle 2, they won't show up on the iPhone. Rumors are that they are coming, but nothing yet.
Apple's App Store gatekeepers didn't have to approve this application. It is to their credit that they did, especially given some of Steve's prior comments about reading and the Kindle.
I expect the Kindle for iPhone app will sell a fair number of books for Amazon, and maybe even a few Kindles as well. If buying books is not your thing, this site has thousands of books already in Kindle format ready to download. There are also thousands of free books available for the other iPhone readers like Stanza and eReader. In fact, Stanza will now sell you books for money, and like Amazon they have several selections from the New York Times best seller list.
As the market opens up, more choices will appear. That's all for the good. A few years ago it would have been hard to conceive of reading Dostoyevsky on my cell phone, but it has come to pass. Imagine what the next 5 to 10 years will bring.