Having read more than two thousand screens (pages?) on an iPad in iBooks so far I don't think there is a major issue. It is highly unlikely that I would replace the iPad with a Kindle.

My camera is a lot better than my iPhone as a camera, but the best camera you have is the one you have with you. In the iPad I can carry enough of what I need with me without having to carry multiple devices.

I have not bought any books in the iBook store, as the store is not available in my region yet. But I got books from Baen Books, as well as O'Reilly, in ePUB format.

The attraction to me with the iPad is that I have a choice. An open format reader and a choice of books stores. I did buy a book from Amazon on the iPad Kindle app, but haven't started reading it yet, so time will tell.