Hawking is a genius. And a frequent Simpsons character. I am not a stupid person. And have never been a Simpsons character. Even though this book is supposed to explain physics to a general audience (ie. dumb it down), I do believe most of this went way over my head.
Don't get me wrong, it's a really excellent book for science geeks, minor or major. It even won a science book award in 2002. And Hawking does dumb stuff down the best that he can. The entire book didn't flummox me, just most of it.
Hawking went over everything from Einstein's Theory of Relativity to string theory to chaos theory to black holes to worm holes to time travel and star trek (he does have a sense of humor!). I really enjoyed his explanation of black holes and worm holes. He spoke about those in simpler terms that were pretty easy to understand. Time travel was confusing and mostly concluded to be impossible at a macro level but not impossible at a micro level.
With the star trek portion, we ventured into aliens and other forms of intelligent life. His take on that is pretty interesting. Along with this, I've been watching his show Into the Universe and he delves more on aliens there. And somehow makes it more interesting.
I think any form of science geek will enjoy this, even when it flies over their head.