Well, I like the app and registered it. $20. The main feature I'm aware that the $20 bought me was the ability to generate the book as a local PDF file.
I like the app because it helped make sense of the cloud of Google books available of Edmund Spenser's works. I had spent a frustrating time searching from the Barnes and Noble site (to download to my Nook) and from the Borders site (to download as epub to my Sony reader Windows application) and some poking around directly on Google Books. I downloaded several things with titles like "the complete works of edmund spenser" only to find that they were essays about his works, or volume 3 of a five volume set (the other volumes not being available), etc. Using GooReader I was able to work through things and find a good single-volume edition of his works, which I downloaded as a PDF. So the primary benefit I experienced was feeling significantly less frustrated and having actually found wheat among the chaff. Now, whether another person would have been able to get good results with comparable frustration levels using the Google Books interface directly, I don't know. What I *do* know is that I found it quite useful. I am aware it gave me a PDF and not an epub, but I will convert it myself.
Obviously my primary use of the product is not as "just another ereader." Which is not its primary claim of value, I don't think. But I do like the ereader interface and I even like the animated page turning. De gustibus non est disputandem.
I did not encounter a case in which GooReader said a book was full preview and the book was not full preview. But I haven't accessed a lot of books yet. Mainly I was weeding through chaff to find the wheat, so I actually accessed only the winners.