Not Nearly as Good as it Could Have Been
I found A Scanner Darkly to be a bit of a disappointment. It should have been a really great movie, but at the end, it wasn't anywhere near what it should have been. Before I continue, I should note that I saw this one on an airplane -- not the best viewing conditions. So take this critique with a grain of salt for that reason.
I found that the story and the acting was very good. This is one of those rare instances where the presence of Keanu Reeves does not ruin the film. Heck, I was surprised to see that he actually turned in a decent performance, almost as good as the one from Bill and Ted's Excellent Adventure. And the other actors were quite good too, all far better than usual. Though, it does speak to the quality of the casts' talents that they play stoners better than other roles.
However, it wasn't long into the movie that I started to get distracted by two things. Firstly, it seemed that the sound engineering was botched. The actors were often speaking in low, conversational and meditative tones. The voices came out all mumbled and muddy. This could just be the result of the poor audio system on the plane, but the other movies I watched did not suffer the same problem.
Secondly, I found the faked rotoscoping to be extremely poorly done. That distracted me a lot from enjoying the film. One obvious problem was that not nearly enough polygons were used. This introduced a lot of edge and texturing artifacts into the film and consequently made it look ugly. Then, there was the artistic choice of just using flattened colours sampled from the filmed scene as the textures. Due to that choice, there was no artistic creativity introduced into the movie through the rotoscoping. It was simply the filmed scene flattened out. Compared to other rotoscoped animations, it was boring. Considering how much hype was made about the style, I was severely disappointed on that front. If you want to see some really cool work done with rotoscoping, check out the animated series Delta State. Rotoscoping can be used well, but A Scanner Darkly shows that it can be used badly too.
In the end, had a decent job been done of the audio and graphics, this would have been a great movie. As is, it is barely worth watching once.