There is also the issue of incident light vs reflected light. When you read paper or e-paper technology, you are looking at light reflected off the object. When you look at a tablet or computer screen you are looking at incident light - that is looking directly at the source of light. Some people find looking at a computer screen for too long gives them sore eyes, however they can spend hours with a book.
Another issue concerning incident vs reflected light is quickly switching between viewing the device and something else. I've found that switching between looking at an LED or LCD screen, and another object requires a small time of adjustment in my eyes. Switching from paper to looking in the distance demands much less latency. A common musical scenario would be trying to keep an eye on both the conductor and score.
A lot of e-book readers use e-paper, you actually need light to see what is on the screen. There is the added advantage of battery life because devices based on e-paper expend their energy turning pages as opposed to displaying the image. Notice reviews comparing e-book readers often display battery life as number of page turns, vs hours.
I would be really interested in a large format book reader. Something practical to display music like a 9x12 display. Even better would be a double width display for a two page spread, like a real book.