What's good about A Scanner Darkly: love for the Philip K. Dick source material, some surprisingly appropriate acting from Robert Downey Jr, Woody Harrelson, Winona Ryder, and even Keanu Reeves, and certain key moments where the animation evokes the confusion and unreal moments of drugs.
But I see a failing that is sometimes attributable to recreational narcotics: an unfortunate tendency for the imaginer to apply too much profundity to their personal hallucinations. More simply put, an enlarged sense of self worth = ego.
You don't even have to take drugs to notice this. Ever try to explain a particularly meaningful or vivid dream to someone, only to have them respond with complete disinterest? Or had someone recite a mind-bending dream of theirs, to which you feign interest because frankly, it's completely moronic? This isn't because your dreams are more interesting, it's simply because the only person who thinks said dreams are so awesome is the dreamer. Maybe flights of fancy in the mind are often only intensely relevant to those intimately familiar with the source material.
Now you might understand my first issue with A Scanner Darkly. Due to drugs, you have folks that only exist in their own paranoid personal worlds, completely cut off from society, with much more screen time dedicated to their thoughts than is warranted. A car breaks down. Oh my gosh, it's a conspiracy. Someone sold me a bike with fewer gears than I originally thought. Oh my gosh, I was so intentionally ripped off.
What's ironic is there actually is a conspiracy going on. But the people involved are too mentally impaired to figure it out. Yet none of the conspirators would be so obsessed with these stoners if it weren't for the drugs.
The second negative is how the animation technique is sometimes strangely limiting. It's rarely used to visualize the trippy hallucinations of bugs and aliens. It unfortunately disguises the movie's ability to comment on real life. There's a powerful core of ideas here - a society infiltrated by surveillance, a war that has blurred the line between friend and foe (to the point where people spy on themselves), and how the source of Substance D is the power fighting it. But much of this timely social commentary I nearly missed as I was distracted by the animation.