Having re-read this thread, I'll say I've come to use my SGT exclusevly when I read. So far I've read a half dozen or more books, some rather thick, and sometimes for hours at a time. All with no more eyestrain that I would have had with dead trees.
However, this thread has convinced me to try using black background with grey or some other color text, not for the eyestrain aspect, but for saving battery life. It takes a lot more power to light up the whole screen, than it does just the text.