I saw The Martian last night and loved it. I had read the book in June and think it's a great adaptation. The story is the same, but the story telling is a little different so that the things a movie can do well it does while the things a book does well it does. You can enjoy the film just as much having read the book or not, and if you want to read the book afterwards there will be more things in it that will keep it interesting. I knew what was going to happen while watching the film and was on the edge of my seat the whole time with a big smile on my face. They nailed this.
Matt Damon is an astronaut stranded alone on Mars. He has to overcome all the various ways Mars is trying to kill him using his wit and determination. Science itself is practically a character in this film. I'd avoid watching the trailer, it gives away too much. The best description of the book I've seen is from xkcd:
It's Ridley Scott's best film since Black Hawk Down. Drew Goddard did a great job on the screenplay adaptation. The scenery on Mars is gorgeous. Matt Damon is great. The movie is thrilling, emotional and funny. I found it really refreshing that everyone in this story is smart, really smart, and no one is stereotypical socially-inept genius that's in every other film. So far it's my favorite movie of the year. Go see it.
Update: Vox talks about 5 of The Martian’s boldest scientific twists, explained.