Reduced Trigger Pull, Increased battery life if using a E-Grip.
Using too large of pin size can cause saftey to not engage.
When I first got the hairpin kit I was using a stock trigger on the A-5. It greatly improved the amount of shots I could put out per second, by reducing the distance of the trigger pull, and allowing me to trip the sear a little easier because of the aggressive pitch of the sear caused by the pins.
When I purchased an E-Grip I could tell that the pins greatly reduced the amount of pull that was required to make the grip cycle. The pins also help conserve battery power, due to the fact that the sear is much easier to trip
So worth it! If you have a stock trigger and want to up the rate of fire that you can put out, or even if you have an E-grip, or a response, these pins are so worth it, they help increase your efficiency and rate of fire. They are worth the money