It's not obvious to me why this book was such a bestseller: after all, there are plenty of popular science authors and books out there. It is, perhaps, an example of the cult of celebrity, certainly not eschewed by Hawking. "Physically crippled with a brilliant mind perfectly intact" is a stereotypical description, but not an inaccurate one.
As for the book itself, it's mixed. On the one hand, I find Hawking's explanation of the Uncertainty Principle rather unhelpful. To the layman, it would suggest that the impossibility of simultaneously measuring position and momentum to arbitrary precision is an experimental artefact, rather than an intrinsic feature of quantum mechanics. On the other hand, his explanation of Black Holes is altogether more satisfying. In particular, his description of light cones "tipping over" at the event horizon is compelling, as is the explanation of evaporating Black Holes (also known as Hawking radiation, though never referred to by that name in this book).
The first half of the book is fairly standard content and relatively uncontroversial. The chapter on evaporating Black Holes is "common sense" based on Hawking's arguments. On arrival at Chapter 8, however, there is a gear change as Hawking moves into imaginary time and the no-boundary universe. The first time I read this through (many years ago) I momentarily thought I saw what was going on, then had a headache. This time round, it was less painful though still unclear. By definition, you can't "see" imaginary numbers in the world aroung you in the same way that you can see real numbers. (In fact, you only ever intrinsically see natural numbers; when you divide something in half, you have two of them; when you compare things you may see fractions, or even, in the case of a circle, the ratio pi. And have you ever really seen "zero"?) Mathematically, however, imaginary numbers just extend the number line into a plane. In relativity, three-dimensional space becomes four-dimensional space-time, but the time component has a different (and imaginary) metric - ic. So in a way, space is imaginary time, and time is imaginary space.
As well as physics, Hawking is keen to offer some philosophy, a subject which he justifiably observes has become more detached from science over the past two hundred years, due mainly to the quantification of science and the mathematical requirements to study it. He is rather dismissive of the anthropic principle (that the universe is the way it is because we can observe it: if it were any other way significantly, then we wouldn't exist in it). He condenses a discussion about the arrow of time considerably, and this makes that chapter unclear, though one could produce enough material on that subject alone for a book. He is truly atheist where Dawkins is antitheist: to Hawking, God might exist, but there is no requirement for him to do so.
The book is rounded off with cameo biographies of Einstein, Galileo, and Newton. It's not clear to me that Hawking must be carved onto the physicist's version of Mount Rushmore alongside them, but mainly because Einstein was really the last "solo" physicist and all significant scientific research is conducted in teams these days. It's a shame that the book has no bibliography as I'm sure Hawking could have provided pointers both to "easier" reads and to books more focused on some of the areas he touches on.
Over-rated? Perhaps so. An achievement? Undoubtedly. At the very least, this book gave physics and big science "the oxygen of publicity" during the late Thatcher years (remember Ken Clarke's description of the European Space Agency as "a hugely expensive club"?), and even if many people bought it and failed to read it cover to cover, it is still an icon today (witness its appearence in the film Harry Potter and the Prisoner of Azkaban).