You lose details and some character development when you translate a book into a movie, just because everything has to be done with visuals and dialogue, but in some ways the movie version smooths out flaws in Crichton's story. Crichton builds things up tremendously well until the tipping point where the fat guy starts making trouble and the visitors break down right next to the T-rex enclosure, who can now get out because the electrical fences are out. But then his story starts breaking down.
He is able to keep you interested enough to keep reading, but it is like he tries too hard to do so. After the kids and Dr. Grant get stuck out in the woods with no choice but to walk back through dangerous dinosaur-island, and everything else starts going to crap, there begins weak danger-at-every-turn writing. They have so many near misses with the Tyrannosaur that it becomes cheesy.
In this way the film takes the upper hand. The movie is exciting and you feel the danger, but it doesn't break down into cheap action-film writing. (At least not as bad as the book does.) A small detail that I find irksome in the book is that it is specifically listed the 15 dinosaur species that the island's geneticists re-engineer, and then later a 16th species is running around. It is like the author just wanted filler, to take the place of squirrels as mild, chittering creatures, and forgot the rules of his own story.
All in all, my favorite character in the story is Ian Malcolm: the brilliant chaos theory mathematician who wears all black and has the cynical (and he turns out correct) view that the very concept of the park teeters on the brink of disaster. The story of a Dinosaur Park, and the way it falls apart because of the problem with attempting something within the scope of scientific possibility but beyond the scope of what humans can or should try to control, is an exciting and enjoyable one.
It should be noted that in the movie Mr. Hammond (the old rich guy who starts the park) is portrayed as a lively, happy old man. In the book where we get more insight into his character, it is seen that he is a foolish old man who tries to overstep what should be done just because it can be done so as to bring dinosaurs back from extinction. Then his idea is to keep them under control in his park so that he can make billions on the novelty. His pride and desire for riches get the better of him, and in the end his dinosaurs eat him.