It's really difficult to not make a comparison here to The Last House on the Left (and even Duel and The Hitcher in certain respects), though David Hess's maniacal character of hitch-hiker isn't the only real despicable person here. Sure, it borders on exploitation status at certain points with some graphic scenes, it's got an intriguing enough plot that isn't too transparent, and some pretty good cinematography (with one glaringly bad exception).
A couple traveling the California countryside end up picking up a hitchhiker. While things are already a bit strained for the Italian journalist and his young wife, the addition of this third person changes each of their lives through a pretty fucked up ride through the state.
I think what makes this movie is the strange dynamic between the characters. Hess inherently comes off as a real sleaze (which, apparently he's a really nice guy in person. Really.), but there's a lot more tension going on between husband and wife, and an interesting choice for some minor characters that come into play. The movie provides a wild card that you really don't know quite how it will play out until the end. There's some bad dubbing and occasionally dated music (afterall, it was shot in Italy in the 70s), and tends to go on a bit too long, but overall it really is a good performance by Hess as the madman and Italian actor Franco Nero as the husband.
