Here are some facts from John Lott and More Guns, Less Violence:
"Our findings are dramatic. Our most conservative estimates show that by adopting shall-issue laws, states reduced murders by 8.5%, rapes by 5%, aggravated assaults by 7% and robbery by 3%. If those states that did not permit concealed handgun in 1992 had permitted them back then, citizens would have been spared approximately 1,570 murders, 4,177 rapes, 60,000 aggravated assaults and 12,000 robberies. To put it even more simply: Criminals, we found, respond rationally to deterrence threat."
"We also collected data on whether owners of concealed handguns are more likely to use them in committing violent crimes. The rarity of these incidents is reflected in Florida's statistics: More than 300,000 concealed-handgun licenses were issued between Oct. 1, 1987, and Dec. 31, 1995, but only five violent crimes involving permitted pistols were committed in this period, and none of these resulted in fatalities."
http://online.wsj.com/article/SB841185795318576500.html