It is what it is.
These films are, by most accounts, terrible because of how sick and twisted they are. But isn't that the point?
I'm a horror fan, and in many subgenres of horror, you need to learn to like 'bad movies'. Horror films go for an entirely different audience response than any other genre in that they CULTIVATE fear, shock, and disgust, rather than avoid it. In the context of contemporary society, where it's becoming harder and harder to scare audiences simply by having a man in a mask say 'boo!', it's impossible to make a film that is both truly horrifying and also create a film that is easily digested and appreciated by audiences and critics that don't normally go for that sort of thing.
I was very satisfied by the shock factor, gore, and the suspenseful scenes. The acting of the girls isn't anything to write home about, but it doesn't have to be, since they spend most of the movie with their mouths grafted to one another's anuses. Dieter Laser is incredibly hammy, and his most cringeworthy scenes are almost endearing in how disturbed and gleeful his tone of voice is.
If Tom Six has anything going for him as a director, it is the sadistic glee with which he tortures and defiles the characters in his films. The Human Centipede crosses the line, then pisses on the line and runs all the way around the world so that it can be crossed a second time. It's Shocksploitation at its very best, beaten only perhaps by A Serbian Film in this genre.