A Scanner DarklyI hadn't seen A Scanner Darkly until yesterday.
When the film was released, the critics were fairly harsh (67% favorable at Rotten Tomatoes). And that number's probably fair, if you're not a fan of Philip K. Dick.
However, and this is a big however, if you are a fan of Philip K. Dick, the film is a masterpiece. I can't imagine another film ever conveying the atmosphere and feelings of his work as accurately as this film. Dick's writing at this stage of his life had become increasingly paranoid, and Linklater's decision to use interpolated rotoscoping was nothing short of brilliant--it perfectly conveys the flickering sense of reality that Dick was both feeling and writing about.
Blade Runner, to me, was a better film (and my favorite science fiction film ever), but it widely diverged from the story by Dick that it was based on. So if you want to see a very true, very faithful representation of Dick's later work, then A Scanner Darkly is what you want to see.