I really liked the way A Scanner Darkly was done. I felt it did a good job of keeping true to the book, though it had to take some shortcuts for brevity. The visual style is creative and not distracting; after watching it for a while it felt “real” which was sort of an odd sensation. Having read the book it “made sense” what was happening, but I think if one approached the movie without that background, it may well be entirely impenetrable; there is just too much context missing, and too much immersion in the story to really get a sense of what is going on. In that regard, the faithful telling may be a bit too faithful; the movie serves only as a clever set of illustrations for the book, rather than a recreation.
On the other hand, maybe that’s the point. If it doesn’t make sense and everything is sort of odd and hard to understand, maybe that’s the appropriate feeling for the story, the same struggle Fred/Bob goes through in trying to piece together the story while somebody else is pulling the strings. Who knows.