This is a really great idea for decorating and look great. I bought a couple and may buy more.
Likes
* It can really dress up a drab wall. Much more eye catching that just a bookshelf or photo.
* It's easy to install. Just need 3 screws.
Dislikes
* You have to be choosey with the type of books used. The ones I planned to use were too deep and too heavy. The whole thing sagged. The pictures on the Umbra site show some with a whole stack of books. I wasn't able to get more than a couple of hardcover books on it, without the sagging occurring. Wish they had used slightly thicker gauge metal. For the price charged, they could afford it.
* The single version has 2 mounting holes one above another, so it offers more support. In this double version, it has 3 holes, just slightly above the shelf, in a horizontal row. This might limit the weight bearing capacity.
* The bottom support book only provides support if other books provide weight on the wall side. It also assumes the very small support "hooks" stay engaged on the book cover. If a guest decides to pull a book down to look at it, it is very likely the whole stack will come tumbling down. This should be considered a decorative solution, not a way to store books you will frequently use. Be ready to yell "don't touch those" if you seem someone reaching for them. I would not put anything breakable below these.
Despite some gripes I really like these and they look great on the wall. I think there are some things they could do to improve them, but overall I'm satisfied. I'm recommending them.