Great premise, poor execution. 
 On the cover of the edition I was reading, someone refers to Philip K. Dick as the "poor man's Thomas Pynchon." I'm afraid I have to agree (for this novel, at least), and I don't mean it as a compliment. In Pynchon's druggie underworlds, the narrative is crafted with subtlety shifting, spiraling language that always eludes any straightforward interpretation; in A Scanner Darkly, the first half of the book seems to be an elaborate over-explanation of the basic premise that's already abundantly clear to any non-drugged reader about 20 pages in. I get it. His brain is split. This could have worked as a 10-page story or (I haven't actually seen the Hollywood adaption, but I imagine it's much better) a movie script.