Purely based on the merit of Abraham Pais' contribution to this work, this is a five-star book, even if nothing else be considered. Stachel once remarked about Pais' book on Einstein, "Subtle is the Lord," that everyone should read as much of it as they can. The reason is clearly that Pais has a wonderful grasp of the technical aspects of physics, and possesses a high degree of competency within the discipline of history, that he is able to canalize an enormous amount of content and detail into an easily digestible piece of literature. That's his brilliance: converting complexity into simplicity, and organization via insightful reflection. Such is the case with his article in this work. In fact, anyone who has read a modest amount of literature on Dirac will instantly become keenly aware of just how insightful this essay is. On top of that, Pais knew Dirac, so his perspective is complemented by the authority of experience. Therefore, being accessible to the layperson and valuable (because of copious citations) to the scholar, I have the highest praise for the essay.
The rest of the book isn't bad either. Hawking does a nice job establishing some context for the book. The second chapter on antimatter is a little on the sorry side, because of the lack of citations and the misleading history it presents. (The author of this section is, quite obviously, a physicist without training in history.) The conceptual presentation on antimatter is fine, though I have seen better. The remain two chapters can be very valuable. The third chapter, on monopoles, can be understood by the undergraduate who has gone through Griffith's "Introduction to Quantum Mechanics," and may even be intelligible to the layperson (but maybe not). The last chapter on the Dirac equation, because of ideas that would probably be new to the undergraduate in physics, such as Clifford algebras, may be limitedly intelligible, but there is still value in the text, beyond the mathematical exposition.
I definitely recommend the this book on the basis of the section by Pais, and the rest is worth thumbing through to see if it is of additional value to the reader.