It does seem to vary by book...
My father-in-law had eye surgery a couple of years ago and can no longer read normal books for more than a few minutes - only the large print ones, which are harder to find. I considered getting him a Sony Reader because of the resizing in any text... but after using it for a while, and while I *love* it myself, I'm not getting him one for Christmas as planned because I can't guarantee he can just hit the size button a couple of times and read to his heart's content. Too bad, he always loved to read.
For the "classics" it's fine, because I format them to a font size that looks fine for me in "small" ("medium" late at night), and fine for him in "large"... but he likes to read new stuff.