Amazing letter. His thoughts and predictions were spot-on, too; Blade Runner definitely changed how we approach science fiction, and had a massive influence on the genre (and even works not of the genre) in a way that Philip K. Dick's works by themselves could not have.
It's also interesting how the dark and gritty style presented in Blade Runner is still directly influencing present-day works. The new Battlestar Galactica springs to mind. When it comes to science fiction, there's just something about this style of story-telling that allows the reader/viewer to relate to it much more easily. It bridges a gap, presenting the strange and crazy ideas found in sci-fi in a much more grounded, realistic way.
I don't think we've seen the pinnacle of this style of sci-fi in film and television yet. Blade runner continues to revolutionise our concepts of what science fiction is, and what it can be.