We are far beyond ordinary humans!! Those of us in CP9 have been in training for as long as we can remember. We honed our skills to serve the government and overcome the limits of the human body. We gained six superhuman fighting skills through that training.
— Lucci introduces Rokushiki.
According to Lucci, those who mastered the Rokushiki will have the strength of a hundred men.
The overall strength of a Rokushiki user is measured by Doriki, which Fukurou can use his "Te-awase" to calculate with. The higher the level, the stronger each technique's potency becomes. Most of the users have their own variation of each technique, to increase their versatility and originality. Also, certain users tend to be more specialized in one technique, such as Fukurou with Soru, Jabra with Tekkai, Kaku with Rankyaku, and Rob Lucci with Shigan.
Rokushiki is the main style of combat for the World Government's secret CP9 agents, but some Marines have also demonstrated its usage. Koby used Soru against Monkey D. Luffy,Vice AdmiralsMomonga and Dalmatian used it to confront Luffy at Marineford, Momonga also used Geppo against Luffy, and Dalmatian also used Shigan against Luffy, Vice Admiral Vergo used it on several G-5 marines on Punk Hazard, and Captain Tashigi used it during her confrontation against Monet. The (non-canon) former Marines Z and Binz are Rokushiki users, with Z being able to master it and Binz seen using at least Soru. It is unknown exactly how much Rokushiki is spread around the Marines and how widely known techniques are, but it seems the most commonly known technique is Soru.
Outside of the Marines, there are also others who learned the Rokushiki techniques, or adapt their principles to create their own, similar techniques. The (non-canon) World Government hating pirate Byrnndi World was somehow able to learn the Soru and Geppo techniques.Monkey D. Luffy utilized his Gear Second mode to grasp the concepts of Soru, gaining an advanced version of it, while Sanji gained an advanced variant of Geppo that he can utilize both in midair and underwater.
Z had mastered the Rokushiki style, despite not being seen using any techniques.
Each of the Rokushiki seems to focus on certain aspects of physical perfection in martial arts, Soru for speed, Rankyaku for kicking strength, Shigan for arm strength, Geppo for jumping agility, Kami-e for flexibility and Tekkai for endurance and durability.
According to Oda there is a joke eighth Rokushiki technique called Chokkai (直貝,Chokkai?, literally meaning "To Bother"), where the user relentlessly bugs their opponent.
↑SBSOne Piece Manga — Vol. 53,
Oda states he can use the Chokkai technique.
↑Vivre Card - One Piece Visual Dictionary (Card #377),
In the online revision of Lucci's card, the wording is changed from "Also learned 'Haki' recently," to "Also strengthened his Rokushiki-related 'Haki',".
↑Vivre Card - One Piece Visual Dictionary (Card #???),
A vivre card draws a connection between Haki and Rokushiki, with the headline stating "Material arts based on 'Haki' also exist" and the picture description stating "The mastery of 'Mantra' and 'Rokushiki' and the learning of Haki make use of a similar training."