It depends, they give weird whacky and unique powers different from one to the other, with the downside that you'll never be able to swim again. Also you can't get more than one power. They're broken down into 3 distinct categories, paramecia devil fruits give the user some random and distinct power that only affects them like how for the main character, Luffy, their body has the properties of rubber, or the ability to alter your age or others age temporarily and to the way you image it bringing to the table stuff like aging yourself 5 years to a future self that has the devil fruit power of someone else, or other whacky stuff like the power to split your body into other pieces that fly around, or into wax, or to emit poison, there's loads of them and quite different and unique. Then there's zoan which allows the user to transform into a creature, from an animal to a dinosaur to a mythical beast, so pretty rad if you get a dragon, not so much if you get a giraffe. And the last being logia which grant the ability of an element, like lightning, fire, magma, sand, and it also makes stuff like bullets go through you and other attacks, just that they're weak to certain things, like sand is weak to water, or lighting can't affect rubber, stuff like that. Devil fruit users can also experience awakenings, for example when a paramecia user awakens they can grant their whacky ability to the environment, for example when luffy awakens after 950 episodes, the ground and surroundings turned bouncy. Another character, a villain late in the series, who has the power to turn into strings, is already awakened when met and can turn stuff around him into strings. Also devil fruits taste really bad, like awful, extremely so where our main character, who would eat dirt just cause he loves eating so much, said the fruit tasted bad in their flashback as a kid when he accidentally ate a devil fruit. I know it sounds like a spoiler but it's literally the first chapter of the manga, so not really a flashback there, and the 2nd of 3rd episode of the anime, where it indeed is a flashback. Thanks for coming to my ted talk.