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The 2 advantages of bistable screens like what is used in the Kindle is the lack of back light and long battery life.

Reading on an LCD causes eye strain because of the back light. The simple solution is to invert the colors. For reading a book on an LCD, have the background color for the page be black and the letters be white. This will almost completely eliminate the eye strain. If you have ever seen a Bloomberg terminal, you know what I mean. Staring into a black screen doesn't cause eye strain.

As for the battery life, if the iPad had an OLED then it could last much longer reading a book with an inverted color screen because OLED uses much less energy to display the color black. It uses no energy to display a black pixel.