This weird hybrid of nature footage, first-person narrative and fictive content is unimaginable as a theatrical release today--but such oddball packages (mostly from "Sunn Classics"), and many Bigfoot-themed features, managed to get fairly wide release in the 70s.
People dislike this cuz it's mostly the narrator's phony reminiscences (re-enacted on screen as if they were shot while happening) of his career as a naturist eventually obsessed with that rarest of alleged critters, Bigfoot. We only glimpse the latter in climactic footage of what very much looks like a tall guy in an ape suit. (The famous raw footage of an alleged Bigfoot is much more convincing, if not entirely so.) It's like an especially crudely-crafted episode of "Wild Kingdom," plus a couple actors in fuzzy focus and hairy suits.
This is an oddity, but hardly a fascinating one. Indeed, it's a bit of a slog, with no real payoff. There are better Bigfoot movies. If that's what you're into.