My strategy in these cases is usually "look for lots of facts relevant to this issue and see what stands out". The things that jump out at me from just that page:

• Many American cities/states (and the entire UK in one very interesting case) have instituted or repealed gun control laws long enough ago that we can look at what happens to violent crime before and after the law is changed. In every case that they showed me, at least, places that pass gun control laws see an increase or no real change in their violent crime rate relative to national average.
• 1/3 of incarcerated US felons claimed to have been shot at, scared off, wounded, or captured by an armed victim, but only 1/12 of violent crimes committed in the US ever result in a prison sentence. My interpretation of these two numbers combined is that owning a gun makes it more likely that anyone who tries to commit a violent crime against you will not be successful, and also will more likely be punished by prison time and/or being shot.
• Just 8% of violent crimes are committed by someone visibly armed with a gun.
• About 11,000 murders per year are committed by gun in the US (in 2008), and about 160,000 people (in 1993) claim they've used a gun for self-defense in a situation within the last five years where someone would have died had they not had a gun. Based on these two numbers alone, and probably not exercising as much care as I should in producing such a pithy and easily-repeatable factoid, widely available guns (in the context of American society in the recent past) prevent on the order of three deaths for every one they cause.

The relative lack of facts that would justify stronger gun control laws on that site makes me suspicious, but I don't see anything wrong with the cited sources for any of these specific numbers.