Well as with most replys so far, I have not had to draw yet. But the instructor for my firearms safety course that I had to take to get my CCW license here in OK, said that if we felt like it was needed, then you'd better do it and go to work. He said if we just "flash" it then we are opening ourselves up for a lawsuit. The criminal can go to the police and say that he was minding his own business when you flashed your piece and you can be arrested. The CCW laws here say that the weapon must be concealed completely. Even if you accidentally flash it you can lose your license and possibly be prosecuted. He also said when you "go to work" with you weapon you'd better put out at least 2 shots and more if they are still trying to flee. We asked about shooting someone in the back and he said to tell the cops "Well they shouldn't have turned away when I pulled the trigger. I don't think I could go ahead and squeeze if he had already taken a few steps away and had his back to me, but I practice so that by the time he sees that I have a gun, he also knows it's loaded........ and he will be able to tell by the muzzle-flash.
He went into a pretty good demonstration that when somebody is shot, they usually do weird things. Think about the last time you encountered an angry wasp nest, if they were coming at you, you were probably doing a pretty good dance and swat to try to keep them off you. That's how I pictured his demo since I have had that exact thing happen (with wasps). The human brain will react and make your body turn and spin while your arms flail around trying to ward off some of those wasps, or bullets if that be the case.
I don't want to have to take a life, but if the occasion arrises and I feel that a life is threatened, I will not hesitate to end a criminals career.