It's only an excerpted montage of the book, but 99% of it is Phil Dick's words -- and they chose all the right scenes. Arctor, Luckman, and Barris discovering Donna's roach in the ashtray is perfectly executed. Keanu Reeves really looks too young for Bob Arctor. But Woody Harrelson nails the Barris character.
[[7-9-06: Barris is nailed by Robert Downey, Jr. Harrelson plays Ernie Luckman ]]
Winona Ryder was also very good, even though she says she had no idea what the movie was all about. (See here.)
Richard Linklater (the director) inserted one plot twist toward the end of the movie that I never picked up in the novel. I'm going to have to go back and reread it to see if that was really in there.
By the way, the speaker notes for the Potlatch 14 "A Scanner Darkly" panel, featuring Bruce Gillespie and Grania Davis, are still around at the Potlatch website. Can't pimp this too often, after all the time I spent transcribing and posting the stuff.