The illustrations in this book do a very good job of telling the story. There is good use of color but the story is absolutely creepy. I wouldn't want my child to see another child cut her heart and put it in a bottle. I know picture books can be used with older children but the odds are more young children will see than older children. In my opinion the subject matter of a broken heart is not presented properly for a picture book.
I think you're right, Genie - this book would be better understood by older children, teens & adults than by the young children who are usually drawn to picture books. I do love the illustrations, though. The colors, textures, and shadows really give life to the drawings, and there is a really good balance between full-page, rich illustrations and pages that have more white space. When reading picture books, I often have to force myself to slow down and really look at the illustrations, but these just seem to beg to be examined and enjoyed.
I would agree with both of you that this book falls into the older age range of the Caldecott Award criteria, which includes books appropriate for children up through age 14. That being said, I have to say that I was really touched by this story; the text and illustrations work together beautifully to make a powerful statement about love and loss.