There have been a number of instances where armed citizens prevented criminal acts from occurring. John Lovell, retired USMC, shot and killed two armed men during an attempted robbery at a Subway in 2007 for example. And someone has already pointed out the Charles Whitman case where Whitman received return fire from both police and privately armed citizens.
I’m pro-gun and a gun owner myself, but gun ownership is more about the owner than the gun, as obvious as that sounds. While I support the right of the average American citizen to own a firearm, incompetent gun owners are extremely dangerous. I usually have no qualms with states that have strict laws regarding screening applicants for gun ownership, my beef is mostly regional, seeing as I live in the state of California, which is about as gun-unfriendly as you can get.
At the end of the day though if you asked me if I’d rather be armed or unarmed, the answer is most definitely the first one. I’m not a cowboy by any means, but I have no intention of being denied the right to protect myself and my family. Some people prefer a dog, some people prefer a phone call to the cops and a prayer, but I prefer six rounds of .12 gauge .00 buckshot and an 18 inch barrel.