For a defensive use to be justifiable, the user must reasonably believe his life to be in danger. There are 365 x 24 x 60 minutes a year, or 525,600. Dividing, we find a life saved every 8.08 minutes. Police take up to 30 minutes to answer calls for help. See the recent reviews of the San Francisco Police in the Chronicle, and the San Jose police in the Mercury-News.
Guns are effective for self-defense.
The police there started a well-publicized program of training women to use guns. Guns don't cause crime and violence. You often hear that you are 6 times as likely to kill a family member with a gun as you are to protect yourself from a burglar. This also ignores the fact that most defensive uses of guns don't involve actually shooting. Most often, the threat of a gun drives off the attacker.