I found it to be a great introduction to relativity for me (first part of the book). Difficult concepts are explained in ways that are very easy to understand. I have read many books on quantum physics, and found the approach of this book not to be the best (check out "The Mystery of the Quantum World" by Euan Squires for a humourous and illuminating read). And to be honest I haven't read the bit on String theory yet, although vacation time is coming up.
But yeah, it won the Aventis Science Book prize, received good reviews and is an international bestseller. What more do you need?