5-inch is predominantly a book reader, not a general document viewer. I can format books for a 5 inch screen with an 8-point font that will be readable, if a bit ugly. Since the pixel pitch is a bit finer, perhaps feathering issues will be slightly less noticeable at standard reading distances.
I would like a nice pocketable device, but single-panel 5" screen, while OK, is not really my idea of a good book. Hopefully one day, someone will figure out a hinged/foldable display arrangement that isn't just a concept model.
Bigger readers like the plastic logic and irex seem nice, but I wouldn't carry them around to read books. I like my market paperback size...possibly as big as A5 (device, not screen).
I'll say that
the OLPC 2.0 concept is a good direction for interface and design...though screen tech still needs to be developed further.