From an IMDB user:
"Salesman" is funny in parts, but taken as a whole it is one of the saddest films you will ever see, a document of the quiet desperation of this lifestyle. The directors of the film make powerful statements, but do so subtly, almost unobtrusively, allowing the viewer to fully engage themselves in the almost routine feel of the film. It is a crime that, despite its strong reputation, relatively few people have seen this essential film from possibly the very best documentary filmmakers there have ever been.
It's about a group of door-to-door bible salesman in the mid-60's, and it's just amazing. There's a quiet to the film -- lots of time spent watching the salesman smoke over lunch silently, or cruise around neighborhoods searching for prey, or watching a televised boxing match that night in their motel room. And it's punctuated with the ferociousness of their profession -- the incredibly competitive electricity that ripples between them and other salesman, that dominates the room when they walk into a house. My grandpa was a tractor salesman (not quite the same thing). But I can see bits of him in these people, and bits that are just unrecognizable, alien, and riveting. For me, this seems most clear when revisiting what things were like only two generations ago (this was Walter Scott's great invention -- let's take you half a century back and show you just how f'ed things were). My parent's past really is another planet.