Even when I abandon a book I picked up on KU, I page through it to the end. Because just because I didn't like it doesn't mean that it's not still a product of hard work and dedication on the author's part. So they deserve their however-much-KU-pays. They do generally get two stars when I'm done, though. I don't typically post reviews to Amazon, primarily because there's a subset of trolls that get reviews taken down if they don't agree with them (and I've found the worse I dislike a book, the more likely my review is to disappear with nothing but an email from Amazon). So I leave stars sans reviews and if I have feedback for the author I try to contact them directly. That said, most of my ratings are 2-4 stars. I only give one star to books that merit the title of crime against the written word, and five stars are reserved for the books that have zero flaws. So unicorns, basically. Three stars are for books that I found enjoyable enough but ultimately forgettable.