It could only have been better if we'd gotten to see trailers for upcoming SF movies instead of depressing award-bait.
Being me, I'm going to nit-pick some the tech and plot stuff. But since it's opening weekend, I'll hide the spoilers.
Translating from book to film always involves changes. Some using visuals instead of internal thoughts, some taking advantage of the medium, and a lot of compressing a long story into what can be shown in 2.5 hours. The Martian also bent some for the sake of giving stars the most dramatic moments.
The biggest change was cutting most of the disasters Watney faced in the book. I figure that was a combination of not having the time and not being able to explain the technical fixes to a general audience. They did a nice job of working around them so the book-readers could pretend all of those had still happened off-stage. For example, between starting drilling on the rover and arriving at the MAV, we never see Watney send a message to NASA.
The part I hated most was at the very end. The Iron Man option is joked about in the book, just a typical example of Watney lightening the mood at tense moments. The movie . . . actually had him cut his suit to fly around on the jet of escaping air. So first, he'd pass out in seconds. Second, it wouldn't produce much thrust since there's no nozzle. Third, he'd barely be able to aim it. Fourth, since he can't aim it through his center of mass he'd promptly go into a spin and go nowhere. Well, Hollywood had done lots of good physics up to then, I guess they had to burst out with some cinematic bullshit.
The real driver was likely to give Matt Damon a decisive action beat at the climax of the movie instead of being carried about like a sack of potatoes. Presumably Commander Lewis stepping forward to displace Vogel on the retrieval also reflects the relative star power of the actors.
The movie spent a lot of time showing NASA doing stuff to simulate Watney's situation, including drilling holes in a back-up rover. In the book Watney talked about that but putting it on screen was more effective.
A small example of the movie doing better was Watney's response to being told to watch his language when texting NASA. In the book he responded, "Look, boobies -> (.Y.)" The movie just showed all the controllers leaning back in shock followed by the Administrator apologizing to the President. Everyone's imagination concocted something much more offensive and effective that way.
The long shots of Mars scenery were gorgeous, and effective at demonstrating Watney's loneliness.
Alas, one of my favorite lines from the book didn't make it in:
Mitch shrugged. “But if I wasn't willing to take risks to save lives, I'd...” He thought for a moment. “Well, I guess I'd be you.”
Don't take my carping the wrong way. This is still a great, great movie.
EDIT: Something I forgot to mention originally. The movie added an epilogue with our heroes watching the Ares V launch. Tied up things nicely, including putting in follow-ups to some book bits that had been cut, such as the quid-pro-quo of having a Chinese astronaut on the mission and Beck and Johanssen pairing up. Watney's speech to the As-Cans was nicely done.