I just read this article talking about how the Amazon Kindle could be even better. He has a telling comment on the economics of the Kindle -- how many books you need to buy to make a Kindle pay off.
As an early adopter of the Sony eReader, I can say that the Kindle has a long way to go, because it hasn't really resolved the fundamental problem the eReader faces, which is that it isn't better than a book.
It's different than a book, and has some advantages and disadvantages, but different is not going to make people change their ways. It's not cheaper than buying paperback books, it's not the right format for replacing hardback books, it's fragile (when was the last time you broke a book?), it can run out of power. Against that you have the convenience of being able to order books wherever you are and reducing the storage requirement of books.
And there are business-model differences, too -- once you've read a book on your Kindle, it isn't worth much, but a physical book can be loaned to a friend, resold, or traded in. So, intrinsically, an electronic copy of a book is simply worth less than a physical copy, but typically they cost more than a standard paperback.
I think there are markets for an electronic book, but it's not really a consumer market right now. A larger-format (8x11") e-book oriented towards college students would be a much better device -- if students can buy their textbooks for $25 instead of $50, the device would pay off in a couple quarters, not to mention getting rid of 50+ lbs of books they have to tote around. But I don't see your day-to-day paperbacks getting replaced by e-books until they get a lot more cost competitive.