The Mane Mane no Mi is a Paramecia-type Devil Fruit that allows the user to turn into a physical double of anyone he or she has touched, making the user an Imitating Human (マネ人間,Mane Ningen?). When the user changes, they also imitate the copied person's flexibility, strength, and abilities, aside from those bestowed by a Devil Fruit. They can change to someone else by touching their face with their right hand and switch back to their normal state by touching their face with their left hand.
The fruit allows the user to become a flawless copy of someone they touch with their right hand, in both face and body. It also allows the user to take on strong opponents with their own strong copied bodies and abilities. Since they mimic a copied person's strength and others that means they also possibly gain that person's weaknesses whatever they might be. Apparently, Bentham has perfect recollection of every person whose form he has ever copied. Though whether or not this may be due to the Fruit's power or merely a product of eidetic memory is unknown.
The fruit also has the power to make the user remember the face of anyone they touch. Of course, the user cannot change their appearance into a person whose face they have never touched, making it impossible to change into someone that the user has never met beforehand. Apparently, the user can manifest several features belonging to different people at the same time, though this mostly comes off as comical and affords no strategical battle advantage whatsoever, making this fruit usable only for purposes such as stealth, infiltration, and subterfuge. As copied abilities must be gained through touch, some stronger opponents are hard to get close to and may keep the user at bay. If the copied person possesses any Devil Fruit power, the user will not be able to mimic this power (a slight exception was found when Bentham was able to copy the face of Chopper's Brain Point hybrid form, which was only able to come about due to the reindeer's Hito Hito no Mi). Furthermore, while the fruit allows the user to flawlessly copy the physique of a chosen individual, it does not allow them to imitate or adapt to the skills or fighting abilities of the person being impersonated. Similarly, the user may also not be able to use their own techniques depending on what form they take; Bentham is unable to use his Okama Kenpo while in Nami's form because Nami's body is not physically conditioned for it. Of course, the user cannot also copy weapons or items unique to the one being mimicked, and whatever items they are wearing on their person remain pretty much the same as they were before they transformed, forcing them to improvise, either by acquiring an imitation or stealing the actual objects (such as Usopp's Sniper Googles). The same goes for a person's mannerisms, as the user still acts as he would normally which can possibly blow their cover.
Kurozumi Higurashi who had this fruit two decades ago used it as part of her deceptive plot to install Kurozumi Orochi as shogun of Wano Country. Notably, she impersonated Kozuki Oden to trick Oden's father Kozuki Sukiyaki into giving Orochi a job in his castle, and 26 years ago, she impersonated Sukiyaki himself to decree that Orochi would succeed him after his death.
Bentham uses his fruit's abilities to create confusion and misapprehension, such as when he transformed into Nefertari Cobra and ordered soldiers to shoot at the citizens of Nanohana and when he transformed into Magellan to allow his comrades to cross the Gates of Justice.
Mane Mane Memory (マネマネメモリー,Mane Mane Memori?, literally meaning "Copy Copy Memory"): Bon Kurei transforms into somebody he has physically touched, taking on their properties including strength and form. While this contributes little to no advantage in combat, video games such as One Piece: Grand Battle! Rush! (and its North-American only sequel, Grand Adventure) depict this move as being able to allow Bentham to mimic the faces of other Baroque Work operatives (Mr. 9, Mr. 7, and Mr. 8) allowing him to adopt range attacks using rifles and cannons. In the Viz Manga and 4Kids dub, this is called Clone-Clone Memory.
Mane Mane Montage (マネマネ合成顔,Mane Mane Montāju?, literally meaning "Copy Copy Composite Face"): Bon Kurei makes a new face by using different parts from his memory of various faces. In the Viz Manga and 4Kids dub, this is called Clone-Clone Montage. In the FUNimation dub it is called Copy-Copy Montage.
World's Funniest Face: In this case, Bon Kurei makes what he considers "the world's funniest face" by combining what he considers to be the most ridiculous and laugh inducing facial features in his collection of copied faces. The purpose of this move is to incapacitate the enemy by causing them to laugh. He uses 90% of his own face and Usopp's nose.
Gomu Gomu no Okama Pistol (ゴムゴムのオカマ銃,Gomu Gomu no Okama Pistoru?, literally meaning "Rubber Rubber Crossdresser Pistol"): A punch while disguised as Luffy. Of course, he cannot use the Gomu Gomu no Mi's power, so it has Luffy's strength, but not his stretching. This is called Gum-Gum Queer Pistol in the FUNimation subs. But, this is called Gum-Gum Lovely Pistol in the FUNimation dub.
Gomu Gomu no Okama Gatling (ゴムゴムのオカマ銃乱打,Gomu Gomu no Okama Gatoringu?, literally meaning "Rubber Rubber Crossdresser Gatling): A volley of punches while disguised as Luffy. Of course, he cannot use the Gomu Gomu no Mi's power, so it has Luffy's strength, but not his stretching. This is called Gum-Gum Queer Gatling in the FUNimation subs. But, this is called Gum-Gum Crossdresser Gatling in the FUNimation dub.
Gomu Gomu no Okama Ogama (ゴムゴムのオカマ大鎌,Gomu Gomu no Okama Ōgama?, literally meaning "Rubber Rubber Crossdresser Scythe): A leaping kick while disguised as Luffy. Of course, he cannot use the Gomu Gomu no Mi's power, so it has Luffy's strength, but not his stretching. This is called Gum-Gum Queer Giant Scythe in the FUNimation subs. But, this is called Gum-Gum Ballerina Giant Scythe in the FUNimation dub.
The Mane Mane no Mi was eaten over 41 years ago by Kurozumi Higurashi. She was killed by Kaido around 20 years ago, and some time after her death, Bentham acquired the fruit and ate it.
With the fruit giving the same strength of the person he changes into, it has not been stated if that strength is set at when he last touched said person or if it increases along with the person getting stronger.
The 4Kids dub mistranslated where Bentham said he doesn't have to touch people in order to change into their appearance.
Despite meeting up with the Straw Hats twice and Luffy on a third separate occasion in Impel Down, Bentham never managed to take the identity of Sanji. On the first occasion, Sanji was in the kitchen on the Going Merry (though in the film he merely stopped Bon Kurei before he touched his face). The second time on the Sandora River he had no reason to after Sanji had defeated him.
Bentham's status as an Okama consigns with this fruit's ability to become any person he makes contact with, regardless of gender.
It's currently unknown if the Devil Fruit mimics only human beings, especially after proving he could transform into Chopper, that, despite his Devil Fruit, he is still a reindeer in essence, and since the fruit does not copy the target's Devil Fruit, can be stated that the Mane Mane no Mi can mimic the target's physical state in the moment of contact.
In One Piece: Pirate Warriors 2, Bentham uses his fruit's powers to disguise himself as Whitebeard in order to fool the enemy in the game's final non-canon scenario.
In One Piece: Burning Blood, Bentham can use his Devil Fruit to transform into either Nami, Robin, or Boa Hancock in order to stun enemies. Interestingly, both Nami and Robin forms are based on their Post-Timeskip appearance.
Additionally, Bon Clay (Clone) (which is Bentham in Nami's form) that appears as a DLC character also uses Nami's Post-Timeskip appearance.