I bought my first Kindle book. I bought it on my computer, and when I started up the Kindle app on the iPad, it magically appeared. While the UI of the Kindle app is not as nice as iBooks, it's not at all bad. I hear the Barnes & Noble one is horrible. From what I can tell, because the Kindle books are created principally for use with their monochrome device, none of the images aside from the book covers have any color. For most books, this isn't really a drawback, but it could be for some. There are two major pros to the Kindle app that has me convinced that I will not exclusively be reading iBooks:
I wouldn't be surprised if Apple released a desktop iBook reader pretty soon. At the moment, I don't even think you can get to the iBookstore from iTunes, only from the iBook app on the iPad.