This reads more like a thought experiment than a normal novel. 'If I were in this situation, what would I do?' The afterword reveals this to be essentially the truth. The thing is, it's a very interesting thought experiment.
While technically science fiction, this is really more in the category of 'technology fiction' - not exploration of basic principles or ideas as figuring out how to apply current techniques an extreme situation. Sounds dry, doesn't it? Yet, despite constant descriptions of how to calculate this and that, the book is engaging and intriguing. Maybe it's because as an I-used-to-be-a-scientist, I liked the calculatory bits. They add verisimilitude. Some of them are a bit questionable, and I didn't take the time to check them, but overall, they seem like credible seat of the pants calculations - exactly the kind a stranded astronaut might concoct. In most cases, I read them, and thought "that sounds reasonable." And Weir is thorough; there isn't much that he tries to get away with by hand-waving. (There is one consistency error, but it's not a big deal.)
Thoroughness and credibility carry a lot of the weight in this novel. But what really makes the book work is Weir's terrific sense of humor. It's that that makes you like his character Watney, and that makes you want to carry on. It's essential in a book that's largely monologue, and Weir pulls it off beautifully. I read this at the same time as a more literary collection by another author. Both books are the same length, but I managed only a few dozen pages in the anthology in the same time I read Weir's entire book. Wry adventure was always more enticing.
That's not to say the writing is perfect. Weir makes some cheap and slightly offensive jokes that should have been edited out. He leans a bit on stereotype for the other characters (the German in particular). And he can't get past the sheer crappiness of 70s entertainment. Generally though, the book is a lot of fun to read - in the way that Stephen Baxter (equally tech and projection-oriented) often isn't. In his afterword, Weir inadvertently suggests a Heinlein comparison, and I think there's something to that.
I picked this book up based on reviews, and I'm glad I did. It's one of the few books that matches its hype. It's a fun read, and one that builds respect for science as craft (not magic genius sorcery). Recommended for everyone, and particularly for young folk just getting interested in science. It's the kind of book I read when I was young, that made me think science was cool, and something I personally could do.