Sophie's World has been a huge success in the Nordic Countries, with good reviews and massive sales. However, I suspect that the majority of the sold books were given to kids by parents who hoped it would magically make their children smarter.
Personally, I found the book rather dull. The fiction part progresses very slowly for 400 pages, then something briefly happens, and then the 100 remaining pages carry on in the same slow pace. The fact part is generic history of philosophy, and it does not benefit from being written in dialogue form.
Besides (spoiler), the "Omigod, we're really just characters in a book"-concept seems logically absurd. In another book that would be no big deal, but in a book about philosophy...
I think those parents would have been better of if they had paired a normal philosophy textbook with a normal work of fiction. (Or perhaps Zen and the Art of Motorcycle Maintenance, which tries the same trick of mixing philosophy with a narrative, but succeeds splendidly).