My niece, who is eight, enjoyed this book. It's a fun introduction to geometry cleverly and skillfully disguised as a story about a triangle who is dissatisfied with his same-o same-o life. My niece was absorbed in the book, and she spent time counting the sides and angles on the shape as it changed from a traingle, to a quadrilateral, to a pentagon, etc., etc. She also got a nice little lesson about liking yourself the way you are.
The only downside was that the story got a bit repetitive, which I noticed more when I read it aloud to the kids. Other than that, though, The Greedy Triangle is a well done math-related read.