I got this from USA Carry, I shamelessly cut and pasted it.

A Realistic look at Conceal Carry

Conceal Carry is for people to have a Defensive position if they are attacked.
Forget all the Dirty Harry and Old West hype.

Many times you will have no idea when an armed bad person will decide to use a gun. You could be in a convenient store when a guy pulls a gun on the cashier and demands money. Now if this happens, you must think, has the bad guy noticed you? If so be careful because he might be keeping an eye on you. If he is aware of you can you pull your weapon and stop him without getting killed? Remember, you have no idea how well a bad guy can shoot. Is it worth taking the chance? Or do you let him run with the money? now if he starts shooting that's another story.

Say he has not seen you, you must then try and determine if he is anxious to start shooting, does he look psyched up, is he nervous, angry? Since he has not seen you, you can more easily draw your weapon and aim. BUT, do you tell him to drop his gun or let him walk out with the money? If you shoot is there any chance of hitting the clerk or anyone else? Naturally if he starts shooting you fire.

Every situation is different, you must size up the situation as quickly as possible and make a good decision.
Remember it's not failure to let the bad guy walk, but it is failure to shoot and hit an innocent person. Do you want a gunfight? How many other people are around? If you tell him to drop his gun and he whirls to shoot at you are you ready to take him down? There must be no indecision once you pull your gun. You must be ready to use it with no hesitation if something happens.

Bad guys do not ask you to face them out in the street at 20 paces for a gunfight.
You also can not be as reckless as Dirty Harry, you must use more logic as to whether or not to get involved in a gunfight.

So leave the Bravado talk in the forums and use good judgement in a real situation