Well, I have my PRS600 and I love it. Unfortunately, the textbook part isn't that great. I am not sure what kind of PDF it is. The way the PDF was created, each section is a separate PDF file, so no file is larger than about seven pages.
It loads quickly at first and gives a readable but rough version. I need to give it a while before zooming and panning is somewhat smoother. Eventually it is OK. I can only imagine how much longer a larger PDF would take.
Luckily, this just would have been a nice bonus but is by no means a necessity for me. I love this reader and I have no regrets upgrading my 505 to the 600.