I watched a police video sometime ago regarding "edged weapons". One section demonstrated how an LEO facing a person with a knife (which could be any other weapon for that matter)separated by about 20 feet and the LEO would most likely not be able to fire his service weapon before that 20 foot distance had been covered by the person with the knife.
Imagine, not being aware of the attack initially and making a concealed draw. Best be moving.
Several years ago, Mythbusters did a segment on the ability to draw, rack, and fire at an attacker carrying a knife (would have been nice to see how it would might have changed if he was already loaded and didn't have to rack).
I like my odds better when the attacker is standing still at a long distance. Now if only we could convince the thugs to be more courteous and stand still when they are accosting us with a knife. I say knife because they won't have a gun, since that would be against the law, which they would never break.