For those who haven't read the book, it's important to know what you're getting into. PK Dick wrote this novel as a way of telling the story of how he and his friends in the early '70s damaged and destroyed themselves with drugs. He tells this story within the framework of a surreal science fiction thriller, but many of the scenes are straight from his own experiences with the unpleasant consequences of people using drugs and disintegrating mentally.
This film does an amazing job of capturing the feel and tone of the book as well as the paranoia, perceptual distortions, and chaos of hallucinogenic overindulgence. Add to that a story that only gradually emerges from the madness, but by the end brings in a lot of heavy ideas such as the existence of free will, whether ends justify means, etc. There is a sense of consequence to what happens in the film, a sense of despair at what has been lost. So this story of drug-addled losers becomes the story of the human struggle for identity and meaning.
This is PK in all his dark and perverse and deeply thoughtful glory.