The cheerful colour matches the room, which is why we chose this. The metal sheet is strong enough to hold a lot of books up. If more than about 2 feet of books, I would get another bookend to insert partway along.
Simple to use: you shove the horizontal part under your books, and their own weight holds the bookend in place. If you take your books down and up frequently, it could be annoying, so you might prefer something that has its own weight or that fixes in place.