Ai Ai no Mi is a Paramecia type Devil Fruit that grants the user the power to generate and control pheromones which may have various effects. "Ai" (愛) is the japanese meaning for "Love".
Strengths and Weaknesses
This fruit's major strength is that it grants the user the ability to to psychically manipulate one's own pheromones as well as the pheromones of others for one's own purposes. The user can exude pheromones from the skin or mouth. With this ability, one could release pheromones that do everything from inducing attraction between subjects (or within a subject for oneself), to exuding amounts of pheromones strong enough to induce sleep, change emotions and draw crowds.
These pheromones may induce a pleasure, or may put subjects into a state of high suggestibility (either of which is useful for limited mind control). Other uses of these pheromones can be used to mark territory (causing people to feel the compulsion to not go into a certain area), or to leave a pheromone path which others can follow in order to find you. The user's pheromones may even increase stamina, slightly stimulate the healing process or (used offensively) cause foes to become disoriented, violent or extremely sick. One can issue a gas that makes people who inhale it more prone to seduction or suggestions. The victims are still in control of their conscious, but have weakened discipline.
A major weakness is sometimes the user may not be able to control/generate pheromones if they are poisoned, and some users may only be able to control a particular gender (usually opposite of user's own. Also, like all Devil Fruit Users, they cannot swim.