Human Centipede is one of the few horror films I can think of where it felt to me like the violence and the central conceit aren't there to disgust, and elicit a reaction of "ew gross", but instead felt like I was watching someone film their sadistic fetish.
I am no stranger to extreme content in horror, I've seen most of the extreme examples of the genre like Martyrs, Cannibal Holocaust, etc. but Human Centipede really left a bad taste in my mouth (no pun intended) in terms of how it revelled in human suffering. I hear the sequels are even worse but one was enough for me!