First, let me say that the film is not as disgusting as the premise. The story involves a crazy surgeon who is renowned for his skill in separating conjoined twins. He decides to create a conjoined being, using three people. Eventually he comes up with a male Japanese tourist, and two female American tourists. They are attached in a chain ass to mouth. Yes, that is disgusting, but trust me, it is no Salò.
There are some good points to the film. First, the American women, Jenny (Ashlynn Yennie) and Lindsay (Ashley C. Williams), are not the most likable duo in the world when you meet them. Once they fall prey to Dr, Heiter (Dieter Laser), Lindsay gets the chance to escape. Whether misguided or not, she decides to return for Jenny... Not for Katsuro (Akihiro Kitamura) however. It is sensible to leave Katsuro behind, as both he and Jenny are drugged. She could theoretically save one of them, but not both. Of course, her chances of leaving alone and finding help are greater than of dragging her friend behind and getting away, but I think most of us would do the same.
Another good point is that Dr. Heiter is just bat-shit crazy. He is not Hannibal Lechter or Lex Luthor. He is a nut bag. Any sensible person would realize that his plan will not work, and it doesn't. There is no magic here. His experiment is a failure. Sure, he was able to sew three people together effectively, but not make a viable being from them. Katsuro, in the lead, is the healthiest, and Jenny, the tail, becomes the most sick and dies.
Third. Pain and physical damage are real. People get hurt in this film, and people get shot, and nobody runs away from injuries that are typically shrugged off in film. Despite the implausibility of what goes on, there is a real world logic to a lot of it.
Ultimately, the preconceptions you might have for the film are not really brought to the screen. The surgery and a great deal of the early violence takes place off screen. Certain aspects are shown to help make the point of what is going on, but it is far less a torture-porn film than most in that particular genre.