I thought it was really good. The first chunk of our hero's time on Mars is so long that I was a bit jarred when they switched to Earth's view of the situation. However, Weir wove things together pretty well. The third act was pretty rushed, and I think he could have done a better job with the Commander's remorse about having to leave a crew member behind. Overall, if I were to give it a "star" rating, it would be 4/5. Maybe even 4.5/5.
The science stuff shouldn't scare you off. I got a "C" in the astronomy course I took in college to meet my science requirement, and I recalled just enough of that to understanding the basic physics in the story.