Bile Fascination: Don't deny it. You watched these movies just to see how far they would take the concept of "sew person ass to mouth".
Dancing Bear: Some people don't care about the quality of the movie, only how ridiculous it is. Felix Clay in Cracked's 6 Horror Mashups Just Crazy Enough to Be Awesome calls it a "Shlock shock horror movie that was made solely for the sensationalism of saying it was actually made [...] The novelty of this movie ended with the idea of it."
First Installment Wins: While the critics weren't satisfied with the trilogy, First Sequence, the first film in the trilogy, had a better reception than Full Sequence and Final Sequence, the other two films in the trilogy. Notably, Roger Ebertrefused to rate the first film, as it was too far out of his experience for him to assign stars to it, but he did give Full Sequence zero stars.
Les Yay: Lindsay and Jenny care for each other intensely, to the extent that Lindsay stops to drag Jenny with her rather than leaving her behind. Jenny's last action is to hold Lindsay's hand as she dies. Lindsay was able to somewhat keep it together until then, but she starts sobbing with grief incredibly intensely after Jenny is dead. It could be read as a lesbian relationship, or at least them having crushes.
There are probably more parodies of these movies than people who have actually seen them (neither film managed a wide release and only played at midnight).
Katsuro's ridiculously poetic threat to Heiter: "The Japanese possess unbelievable strength when backed into a corner!"
Heiter's really pathetic attempt to mimic chicken clucking.
"SWAAAAAAAAAAALLOW IT, BITCH!!"
Then there is the actual Human Centipede itself. It's just three people kneeling with their mouths sewn to their asses. The concept seemed much scarier in your head now, didn't it?
The last 20 minutes or so of the sequel zigzags between this and Squick. It's really hard not to laugh when you see all the victims sewn together because you know this is ridiculous. But at the same time, it's really hard not to gag when they all shit in each other's mouths. But then Martin pukes afterwards, and the next thing you know, you're laughing again. But then the pregnant woman turns out to be Not Quite Dead, gives birth in a car, and crushes her baby's head underneath the gas pedal. Start gagging again. But THENYennie hits Martin in the balls, and shoves a centipede up his ass while he pathetically whines. Start laughing again.
The concept of a "human centipede" may seem ridiculous (to say nothing of the hilariously over the top acting), but that doesn't stop the films from being both scary and disgusting, especially the sequel.
Dr. Heiter's Large Ham tendencies — as cheesy as they are — really do convey that Heiter is beyond listening to reason and raving mad.
Nightmare Retardant: Although the premise is really Nightmare Fuel, the absolutely narmful acting and the efforts made to make fun of the film make it nearly impossible to take it seriously anymore. Perhaps it really is scarier if you don't see the movie.
Oh Lord, where to begin... How about the part where Lindsay is forced to eat Katsuro's poo, for starters?
You'll need even more Brain Bleach for the sequel.
And more than that for the third movie, which Six said "makes the sequel look like a Disney movie". (a review even follows a description of the Villain Protagonist with "Here, I’ll start a new paragraph so that you can take a break to go throw up.")
calls it a "Shlock shock horror movie that was made solely for the sensationalism of saying it was actually made [...] The novelty of this movie ended with the idea of it."
