I have reviewed the PRS-600 and although it's a nice device, the lack of contrast of the screen (due to the touch screen layer) and the fact that the screen reflects badly keeps me from recommending it to avid readers.
The contrast of the displays of the PRS-300 and 600 compared:
The PRS-300 (left) and PRS-600
Two crops (same exposure)
Also have a look at the following pictures, which show you the reflectiveness of the PRS-600 screen, compared to that of the PRS-300 (pictures made from the same position, with the table-lamp turned on):
PRS-600
PRS-300
Two additional images, taken by daylight:
If you really want to annotate your ebooks, or be able to search in your ebook library, the PRS-600 is your choice. But for reading purposes only, I do not recommend the 600. If a 5" screen is big enough for you, have a look at the 300, otherwise you might consider getting the old 505 (6", without touch screen).