If the movie follows the book for the letter it will be:-
1 - way too long
2 - you'll be bored and lose interest because it is way too long
3 - you will be annoyed of the constant repeat problems and obstacles he has to overcome, it gets tedious and after the 15th one you'll think "We get it already, Mars is trying to kill him, get on with the next chapter!"
4 - you will be complaining it is way too long
Because you'll be bored, that automatically equate bad movie in my book. A book's first job is to entertain and hold my attention. The Martian, the movie, in its current form does exactly that, it entertains. It's a great movie. Nitpicking how close it sticks or not sticks to the book is unfair.
Imagine for a second, instead of the current 90 mins of a film on how he overcame obstacles, and 30 mins of other stuff with other actors. Now we have 4 hours of that, 1 of them just watch him drive in the Rover, an hour of him fixing various machines, 30 mins of him moving rocks.
Zzzzzzzzzzzzzz.
Once you get the point across, you move on.