My short review is that this is possibly my favourite Philip K Dick book, and this is a good adaptation of the novel. It might not be everyone's cup of tea - I think some will find it too challenging, and some may feel it isn't challenging enough. However, I think given the trade-off between the fun of having money spent on this project, and the constraints of commercialism, this strikes a good balance, and doesn't betray the original.
It's made in rotoscope, which is animation made by drawing over live footage. Some reviews said this is hard to watch, but I didn't find it distracting at all, and I thought the rendition of the action moved in a controlled and intelligent way between photo-realist and various kinds of non-realism. I think the technique not only enabled the story but enhanced the movie.
This is a story about drugs. Like Trainspotting the characters are out of their heads. That could so easily be awful but it isn't.
The plot was simplified and the motivation of the characters was made more obvious, but I don't think that could have been avoided. The basic trajectory of the story was retained. The main characters were retained.
The sole female character - Donna - was altered hugely however. In Hollywood there are really only two approaches for women, they are good and suffer, or they are utterly evil. I think if they had kept Donna as she was in the novel - selfish, faithless, venal, 'frigid' - then she would have been Hollywood 'evil'. Which would be cracked, because she is adorable. So all her negative characteristics are explained away by a bogus add-on to the plot. Sigh.
Of the male characters Keanu and Woody Harrelson are likeable, funny, stoners. I liked 'em.
Rory Cochrane was good as Freck (actually a composite of several characters in the novel). A virtuoso funny/sad portrait of a vulnerable kindly psychotic. Boy, he's come a long way from CSI Miami.
Robert Downey Junior as Barris however blows the whole movie out the water. He is fantastic! I loved every scene he was in, every sentence. Worth seeing for this performance alone. This is not me saying 'he was sexy' because he absolutely wasn't, he was just really good.