There are a couple ways to look at the effectiveness of 380auto. It's very effective when used against normal criminals...robbers, rapists, etc, probably close to 100% that you'll defend against an attack with the pistol. Normal criminals will run when faced with a gun, even if they have a gun, or it'll buy you time to get away yourself.
Against determined criminals like terrorists, vengeful spouses, exemployees going postal, crazy people, druggies, and other criminals that may not run...380auto has a low probability of bringing them down. But that's true of most defensive pistol calibers like 9mm and 40SW. You'll need a rifle or shotgun to have a high probability of bringing a criminal like that down. Hit placement is key with a handgun and that requires a lot of training that majority of gun owners don't have.