I haven't read it in several years, but my impression was that the ideas were interesting, and the book itself was original, but that the characters started off really annoying and never got any better.

Sophie is really childish, and, frankly, kind of dumb, and she's what, thirteen? She's an extremely unlikable protagonist, especially when compared to the slew of smart, sharp heroines of the same age in other young adult novels. In turn, Alberto often condescends to her, drastically simplifying their dialogue so that she, and, presumably, the average young-adult reader, can understand. The philosophy itself read more like a high-school intro class than an in-depth and thought-provoking study, and fails to give more than general overviews of particular movements or thinkers. Overall, I found it disappointing.