I heard about this just recently and loved the idea. I can be quite particular about the cover of a book. If the blurb sounds good, but the cover doesn't catch my eye, I am more likely to dismiss the book. Sounds silly, but it is what it is.
My library does this. This year I am reading an autobiography of Carla Zambizi. Last year it was about some footballer. That didn't go down so well. Fun idea for a library. Not sure I would buy one though