I downloaded it an hour ago and had a chance to briefly use it. It gives me access to all of the books I’ve already purchased. I don’t see my newspaper subscriptions, but I suspect that will come in a future release.
The interface more book-like than the Stanza Reader. Where the Stanza reader uses touch regions to turn pages, on the Kindle reader, you use the page change gesture of sliding your finger from right to left across the screen as if you were flipping pages. Page changes are fast without the refresh flash you get on the Kindle device.
The iPhone is just about my mobile all-in-one device.