Purely by accident, I read The Martian just a few months before the movie came out. It was nice being able to compare and contrast with the book fresh in my mind. The movie held up amazingly well, IMHO. It skipped or glossed over some bits that had I liked, presumably to cut running time, but nothing that was indispensable or damaged the overall story. Hard sci-fi is so incredibly rare in cinema; The Martian and 2001 are the only decent ones I've seen.