I haven't read the book. I had a roomate who talked about it incessantly. He was a big fan of Dick (

).
I like the Director's cut. That narration is pretty lame.
What, if any, scenes really drive the theme home for you?
Actually that's a tough one, huh? Every scene in that entire film is exquisitely crafted. It's one of the most visually striking and beautiful movies I've ever seen, from the first scene.
Near the end when Rutger Hauer makes it to Tyrell and kills him. That, right there, is the thought.
He met his maker... and was disappointed. So much so he killed him. You could see the anquish in his face as he crushed Tyrell; he didn't want to, he had to. That reminds me a lot of a famous and oft misquoted statement by the center of the post modern thought flame, Nietzsche... (ah yes, good ol' Fritz): "God is dead."
I think the entire movie is an exploration of this perspective on post industrial, post modern life. Not in the blaspheming sense, not just to piss off Christians, but in the sad, desperate sense; God failed us and our feeling of betrayal pushes us beyond God and gods, beyond good and evil, an inevitable yet terrifying station for humanity to check in at.
Of course there is much more. That was only like ten minutes of the movie. But maybe it's better to start there and work our way out . . . ?