The Ruins is a story of conflict between Man and Nature. In this case Man is represented by 5 sheltered and somewhat ditzy college kids, while Nature is championed by a malevolent, mile-wide, super-intelligent, highly mobile, acid-blooded plant. The Plant might also have psychic abilities. After 500 pages, Nature wins.
There were some good things about this book. The book reads quickly. The author makes an effort to connect the character's moral failings with the mistakes they make and their eventual fates. About 300 pages in, there is a nice subversion where the characters discuss how, after the rescue, they will be portrayed in the movie version of the events. At least belatedly, they become aware of what genre they are in.
There are a lot of bad things about the book. One is that about 200 pages in you realize the characters are screwed. The antagonist is completely overpowered, so you don't feel any hope that the characters will win/survive. The characters are not very likable or intelligent, so you don't care that much that they won't survive. The characters are completely passive in the face of the Plant's assaults. They never try to burn or cut up the Plant, even though they have plenty of highly flammable tequila and a good sharp knife. The Plant's Mayan guardians are laughably ineffective, and as far as I can tell their patrols have yet to stop anyone from venturing on to the forbidden hill (see: shoddy ending). The Plant itself, while it has some interesting qualities, isn't scary in a large scale way like nuclear war and zombies are. In total, the Plant has killed about 50 North Americans, about as much as a poorly designed on-ramp. Unfortunately for the Plant, in a month or so the missing person's trail will lead to the hill, and then they'll hire some contractors to pour gasoline on the hill and that will be the end of the Plant.