See what the poster says? Well, nasty, savage, and brutal barely begin to describe this movie. You can throw in violent, blood, gory, and sadistic, and there are probably still some adjectives lacking.
The story opens only a short time after the end of the move's predecessor, House of 1000 Corpses, which I haven't seen. A misshapen hulk drags a corpse through a wooded area. A guy is asleep with the corpse of what once was a woman. Two living women sleep peacefully. Then the sheriff and his deputies arrive to "do the Lord's work." A bloody shoot-out ensues. Some members of the Firefly family are killed. Mom is captured. Otis and Baby escape, get in touch with dad, and then things really get hellish. Even the sheriff who wants to do the Lord's work changes until by the end of the film, he's no better than the Fireflys.
Not that the Fireflys don't have a sense of humor. They take their nicknames from Marx Brother movies, after all. On the other hand, they get their laughs from killing people, which isn't funny at all. They kill because they can and because they like to. It's what they do.
If you didn't know better, you'd swear The Devil's Rejects was made around 1977. It's a perfect re-creation of the exploitation films from that era (The Hills Have Eyes comes to mind). If you like that sort of thing, you'll like this one.