I bought the Sony PRS-600 last year.. unfortunately it was fairly slow (with textbooks it was painfully slow), the unit also had a slight glare to the screen and the Google Free books (pdf format) had several issues the most obvious being poor contrast.
Sony released the latest 6 inch touch screen the PRS-650.. I sold my old eReader and based off many reviews (YouTube demos etc) I took a chance. All I can say is, the latest is nearly perfect.. the screen glare is gone.. it loads pages twice as quick and the contrast issues and some flow problems have all been corrected. The touch is awesome especially for making notes in textbooks.. I plan on getting a tablet early next year but I will always have a reader for well reading
Now if Sony would only fix thier windows application.. maybe they could switch to Silverlight