Pro: Diffrent rev limiter. Set the rev limiter 100rpm under the stock limiter it is a spark cut limiter that way instead of a fuel cut limiter. It is safer for the engine since the fuel cut limiter causes a lean condition.
Pro: I've heard MSD's service/warrenty dept. is badass if you ever have a problem with it.
Con: I've heard they can be loud making a clicking or humming sound.
I still want one anyway you get yours new or used? How much you pay?