I'll just come right out and say it: while this movie is pretty good, it is not as good as the book. The Martian, by Andy Weir, was a surprise bestseller, a terrific science fiction book where even NASA said that the science was spot-on. It is a very funny book too, in addition to all the science and suspense, told in first-person by Mark Watney, stranded on Mars after a mission gets scrubbed. You can read my mini review of the book for plot summary, if you like, because the movie does stick pretty well to the narrative.
The Martian-movie version is a little too long and just not as funny or as suspenseful as the book. Although he does a decent job, I'm not convinced Matt Damon was the right choice to play Watney. It's a tough role, seeing how so much of the story is just Watney, alone on Mars, doing calculations and science-ing the shit out of stuff. The rest of the cast is good, filled by surprisingly big names who don't get all that much to do. I'm glad I watched it but if you have a choice, read the book.
44 minutes ago