A Scanner Darkly is my favorite book of his (that I've read so far). I think because it's a bit longer than most so there's more time to develop characters. It's got such a sad and interesting tone.
I loved the movie too. I was really pleased I read the book first because although the movie is a really good rendition, I feel it would be hard to comprehend certain parts of the movie without knowing the book.
The acknowledgements page after the ending of the book is brutally heart breaking: he mentions all the friends he lost to psychosis and death, due to the drugs they all took together.
I also find that the book remains a fantastic read a second or third time - which I only find worthwhile doing with a few books.
Yeah, I thing that movie is the best rendition of a PKD novel I have ever seen, partly because it is so weird, and partly because it captures some of the black humour of the novel.
