A Scanner Darkly was always my favorite PKD book, though I've not read one or two obvious works from the canon that might one day supplant it when the glaring omissions are rectified. I'm gratified to report that I thought the film was a faithful adaptation - the most faithful adaptation the multiply-filmed SF writer has ever gotten. Very well done, and with a painfully, beautifully poignant ending, and, while I am not among Keanu Reeves' detractors (indeed I admire him greatly for his continued support of so much smart SF), I must say that Robert Downey Jr. stole the show.
Which was not, as some reviewers have asserted, confusing in any way. However, as others have testified, after sitting through two hours of stoner-babble, I did walk out of the theatre with something of a contact buzz. Slightly bug-eyed and thinking"Are they looking at me funny? I'm acting normal, aren't I?" and all that. It was bad enough that I went into a Barnes & Noble to shake it off, where, coincidentally the first snatch of overheard conversation that wafted by me was, "Think of it as an auditory hallucination." Thanks for the synchronicity. That sure didn't help. Thankfully, normality was restored with the very first sip from my one & only drug of choice these days - a double-shot cappuccino from Starbucks with skim milk and two Splendas. And all was as it should be. Welcome to the truly wired world.