I'd read the book last year and really liked it.
I consider the book to be 'hard' science fiction but in a different sense than I usually mean it. All the science and engineering in it is based on actual science, not speculative stuff like warp drives and alien species. There's some suspension of disbelief involved in that if you know anything about the current state of NASA's plans for a Mars visit, a lot of problems that don't seem to have solutions get solved.
Mark Watney gets stranded on Mars by a wind and sand storm much stronger than anything expected. Apparently this is the most unbelievable part of the book and the movie. There just isn't enough gravity on Mars to produce such a storm.
The book spent a lot of time with Mark as he figures out how the "science the shit out of" his predicament. Lots of calculations and Mark talking to himself. Much of this is cut down in the movie and it's just as well. It's one thing to read about Mark calculating how long is Mars grown potatoes are going to last but actually watching him doing the calculations would be a bit tedious.
The movie was very faithful to the book. Mark in the book is a wise-cracking astronaut and I thought Matt Damon did a good job of getting that across. Mark in the book is about the most determined man you could imagine multiplied by 10. This comes across in the movie a bit like cock-eyed optimism but Damon never goes over the top with it.
Back on earth, NASA first tries to cover its ass, making Watney a dead hero. But after Watney makes contact and they realize he is still alive, the powers that be at NASA realize that they have to mount some sort of rescue mission. They try to do it as cheaply as possible in the beginning but as Watney faces setbacks, NASA takes on the challenge whole-heartedly.
Is Watney rescued? Well, the trailers I saw didn't say so you're going to have to see the movie or read the book. As with just about everything else, the movie follows the book pretty closely.
In case you've forgotten, this isn't the fist movie about an American astronaut stranded on Mars. There was this 1964 movie which has more action but doesn't seem as realistic.