I dunno, I think there is more to e-ink than just ambient lighting vs light shining in your face.
For one, in ink, each letter is actually a filled in letter. On an LCD, even good one, it's still a bunch of little dots with spaces in them, it's really only an optical illusion that they look like solid letters to you.
I suspect the quality of the screen is important. On my PSP or DS, which you can actually see the lines between the dots, my eyes start hurting a lot quicker than on my computer LCD monitors.