I use LCDs all day almost every day and if I don't take breaks it can be tiring but I'm used to it. However, I tried long term reading with my with my wife's iPad and I find the big difference is that with the LCD screen, I am compelled to keep it a fair distance away from my eyes to reduce strain. If I do I'm fine. With e-ink, it is almost identical to reading paper and therefore I can hold it at a more natural book-like distance.
I realize comfort at distance can be related to vision but I'm using the same font size and my same glasses on both and there is a difference for me.
If I were buying an Internet/Computing tablet, there is certainly no way e-ink can yet offer a usable interface but since this is strickly for reading only, I have to say that e-ink is far superior to LCD for strictly reading purposes. I also like to read outside in the summer and it suffers no ill effects from brightness or glare.
I will counter argue myself on one small point. A technology that returns us to those tiny, clip on reading lights could be considered a laughable step backwards but at least now with LCD lights, you don't risk falling asleep and setting your bed on fire.
