They make minimal difference, the idea is they clean the face of the pad and remove gases from the pad face. But unless you're driving a racing car on the edge of it's capabilities you wont really notice any difference on a road car. Replacing old brake fluid with fresh and making sure the calipers are cleaned and lubricated will make a much more obvious difference.

I fitted black diamond discs recently on mine, mainly due to the fact they come powder coated in black so won't rust they do them in plain, grooved and drilled. I just went for plain as its a Range Rover not a sports car