I've been relatively happy with the IPS panel in the iPad 1 not straining my eyes. My main gripe is that the brightness of the backlight doesn't go low enough -- in a dark room (i.e. in bed with no lights and someone else sleeping nearby), it would be nice to be able to use the iPad with very minimal backlight. E-Ink would require a book light (which is kind of lame). This seems to be one of the places where OLED would be superior.
It's frustrating that IPS LCD, OLED, and E-Ink all have such distinct advantages, so the idea of having a single device with the best screen in all contexts isn't really possible. I assume OLED will eventually be that.
There's nothing inherent in e-ink that makes it less stressful than a backlit display. Photons are photons, whether they are reflected from a light bulb or the sun to the book and then your eyes, or they are send from a led behind the screen to your eyes.
Some early e-ink manufactures used to claim that e-ink is better than backlit for the eyes, but they retracted those claims because there was no scientific evidence behind them.
You just need to set the brightness of the iPad appropriately, to match what the relative brightness of an e-ink reader would be. You can also adjust the colors, i.e have a sepia background etc.
Battery life and reading in the sun is indeed much better with e-ink devices, though.
