Dick always did have a fascination for the workings of the mind, whether perceiving "reality" (We Can Remember It for You Wholesale, the basis for Total Recall) or identity (Do Androids Dream of Electric Sheep/Blade Runner). This book combines the two topics in a story where many things are not what they seem.
From the back cover: "Bob Arctor is a dealer of the lethally addictive drug called Substance D. Fred is the police agent assigned to tail and eventually bust him. To do so, he has taken on the identity of a drug dealer named Bob Arctor. And since Substance D--which Arctor takes in mammoth doses--gradually splits the user's brain into two distinct, combative entities, Fred doesn't realize that he is narcing on himself."
Dick writes with authority about an addict's mind, lifestyle, and deterioration; as he describes in the afterword, he was one himself. That, to me, is the book's primary strength. Creating and maintaining a nebulous sense of menace is another.
Its weaknesses relate to the passing of time and its length. First, I can believe that the book was important at the time it was published, perhaps even groundbreaking. Given the almost 40 years of SF and scientific/tech advancements since then, though, it comes off a bit flat. I wish I had read this before reading similar novels or seeing movies like Total Recall or The Matrix, among others.
Second, it's inherently less interesting to watch or read about other peoples' highs/trips rather than live one yourself--a little "dude, that's deep, man" dialog goes a long, LONG way. I think the book would have done better as a tight, kickass novelette or novella. It'll be interesting to see whether my opinion changes after seeing the film of the novel.