Almost as good as the book, in my opinion. Definitely something I’d watch again with different friends or something.
EDIT: Okay, perhaps I should elaborate a bit.
The movie is based on the book, and surprisingly enough, didn’t diverge all that much from the source material. There are several parts that got cut and condensed due to time, which is understandable given that there’s so much sheer stuff going on in the book that it’s hard to put it all in a movie (heck, the movie itself is already over 2 hours long even AFTER cutting what they did).
Since it’s based on the book, it’s pretty scientifically accurate with only a few exceptions that I can think of. There’s at least one scene that I can personally say is accurate (specifically, the scene when he hexedits a Linux shared object file in the Mars rover to modify the rover OS to do [SPOILERIFIC STUFF]… I’m pretty glad they kept that in the movie in some form or another, since it was one of my favorite parts of the book). Since they had to condense most of the sciency stuff for time, they don’t go as in-depth as the book when they talk about running numbers, but they kept most of the major number crunches, so there wasn’t too much lost there.
Probably my biggest gripe would be the ending scene, which they modified oh-so-slightly from the book to be marginally ridiculous. If you haven’t read the book, it might not be so bad. But having read the book, I kind of watched it and said, “wha…?” to myself. Regardless, it was still very entertaining. One of the best movies I’ve seen this year.