The Martian doesn't have the best writing. It can feel forced at times, and some of the humor is wince-inducing.
That being said, there are entire chapters that deal with dirt and they're absolutely riveting, so it all evens out.
The movie will be released on Oct. 2, and the book is on sale for $2 right now via Google Play. That's an absolute steal for one of the most interesting science fiction novels released in the past few years, and the fact some of the writing is clunky actually helps with the sense of reality; this is how a goofy-ass astronaut with a sense of humor put in one of the worst situations imaginable would write a journal.
If you're curious about why people are so into this book, this is a great excuse to find out before the movie is released.