As a person with no prior philosophy knowledge, I found the book really informative and enlightening. Since the subject might be boring to most people, I think Gaarder did a good job sprinkling the mystery bits on top; also, the plot twist in the middle of the story was surprising and enjoyable. The subjects began as simple as possible, and grew more complicated as our knowledge grew bigger. The text was clearly well-thought out.
I read the book at a time when I had too much free time and not much on my mind, so I didn't have to go back 5 pages every time I picked up the book from where I left it (I would've if I was reading it at a normal pace). While it's clearly not an encyclopedia, the book is a good attempt at an overview of all the major philosophy subjects in chronological order, and it serves its purpose of informing young readers(or readers inexperienced in philosophy.)
I would've given the book 5 stars if it wasn't for: 1. The lessons sometimes being too long that it felt like a textbook. 2. The feeling that some important philosophers were being left out. Some great names were mentioned but weren't explained. 3. The creepy meetings between a middle-aged stranger with a teenage girl in the early chapters. Sometimes I felt a murder/rape coming, but fortunately it never did.