I haven't seen Irreversible, Men Behind The Sun or The Human Centipede so I can't comment on those. However, I can say that Salo is the reason I will not watch Human Centipede or any other film now that just tries to shock as its main focus. I can't honestly recommend Salo to anybody. I was never bothered by gore and violence this side of being 8 years old but with Salo its not about that. It's the ordeals that those kids were put through and their treatment by the ruling classes purely for their entertainment. It's difficult to explain but the film itself left me feeling depressed and ultimately switched off any interest I used to have of films that purposely went out to shock ie Human Centipede. The only other movie I've seen that I can sort of relate to it is Street Trash. A completely different type of film but it left me with a similar feeling after watching it. Just not a good viewing experience for me. I have to watch Carpenters The Thing to restore my faith in horror whenever a film does this to me.
I'm still a huge horror fan ... just not into THAT type of film.