I finished A Scanner Darkly, which was the trigger for this whole business. I will echo the folks who say the movie managed to follow the book remarkably closely. I enjoyed it.
Oddly enough, I think I might have liked the book less if I hadn't seen the movie first and liked it. If I hadn't known what was already going on, I might have been more annoyed by the characters and their quirks. I don't have much experience to be evoked by reading a description of the substance use in ASD or, say, Requiem for a Dream. Seeing a depiction of it on film gives me a reference set.
I meant to mention in my earlier post on VALIS a quote by Ursula LeGuin gracing the cover, describing Philip K. Dick as an unrecognized, home-grown Borges. cjot and I agree there is a thread of urgency (perhaps paranoia?) running through Dick's work that Borges lacks, but they both incessantly play with the nature of reality, it's true.