Of all PKD's writings this is the worst one to adapt into a movie. It's an insane man novelizing his descent into insanity, and hamming it up on top of it.
I'm not saying it's bad, but it will only hook you if you're a fanboy, which I am.
I've tried to read the Valis trilogy, but I can't work my way through the first chunk because the self inserted crazy becomes really concentrated and uninteresting.
I like Philip K Dick, but he also beat the fuck out of his paranoid reality dead horse by this stage of his work.
This is the worst PKD novel you could ever make into a movie. Even PKD probably wouldn't have wanted it published. In fact, he pretty much didn't, which is why he re-wrote it into VALIS and published that instead.