My controller showed stick drift. After I disassembled it to reach the joysticks and did some tests, I realized that the problem was not in the joystick itself but in the metal rail (white component, inserted into the green slot) that the potentiometer uses to "read" the movement. So I removed one of the two rails from the new joystick, replaced my faulty one, and the controller is as good as new again.
So I did my job perfectly, hence the five stars.