Depends on what kind of modded controller you are getting, many people in the FPS industry consider "modded controllers" as a tool to give you an unfair advantage I.E Triggers that trigger three times for rapid fire effect. Many MANY servers ban the use of said controller.

HOWEVER! there is ONE controller I know of that is MLG approved and accepted by the gaming community, called a SCUF controller.

SCUF controllers only tweak the controllers so that you only ever need to use your own skill in a game, 360 pads have trigger stops so that it shoots a weapon at the PRECISE point in the trigger and stops you there, this leads to faster firing BUT with the use of your own fingers not the programming. There are also paddles on the back that are bound to specific buttons so the rest of your hand has something to press and lengthens game sessions.

PS3 controllers have the paddles but not the stoppers, instead they tweak the sensitivity of the triggers so that if you just tap it, the trigger well... triggers.

BEWARE! The controllers are VERY PRICEY since all of them are handcrafted. You can actually reduce the cost by sending them your controller.

You can find said controllers here: scufgaming.com/

There are many benefits of having one, but it depends on what type you get, for example the ones that GoGetA listed are good ones. I have a modded SCUF controller, which i painted and changed the appearance myself, yes they are pricey so beware