There's no question that this is a valuable book to read. For no other reason than that it surfaces all sorts of feelings in yourself and makes you re-evaluate how you react to any kind of person that doesn't look or act like you.
It's certainly written for teens and pre-teens, and I'd value its inclusion in the appropriate school curriculum. But probably mostly as a teaching tool, and an aid to further discussion. It provokes a lot of thought, so the writing has to be commended to some degree for that. However, I did find the description of many of the children to be somewhat facile. I think it's done deliberately, there's no question of that, and for decent reasons.
But the children often seemed unrealistically described. I don't think most children have such good intentions and thoughtful hearts and minds - then again, I don't have children so I very possibly know nothing! We saw some of the conflicts described, but I would have appreciated Palacio providing some differing points of view. Perhaps bringing one of the "poisoned heart" children around would have helped the storyline. Anyway, the unrealism didn't bother me enough not to finish it! In record time, too.