Even though I saw the movie (so the plot held no real surprises), the book was great. It made me laugh out loud at times, which as far as I recall only Pratchett and Sapkowski managed to achieve. It was well written, I almost couldn't put it down.
The movie was great, something that surprised me as recently Ridley Scott has been making.. well, crap movies, basically. But it was great thanks to the screenplay and writing- it wasn't mr Scott's ;D
Anyway, I'm rambling..
If you like well written, funny and interesting stories, full of scientific and technical details, grab The Martian.