I was an "adult" (older than SC, alas ) when I read the novel in its first printing. Same for the movie. I couldn't put the novel down, and read it hundreds of times over the next five years or so.
GF made a huge impression on me. GFII initially struck me as too long after seeing it the first time. But I had no opportunity to see them again until 1983, when the "Saga" was first shown on TV. That's when (and why) I bought my first VCR. Thereafter I watched the tapes religiously, and came to appreciate II as perhaps even better than GF.