Working with a few defense and LE experts, we ran through this experiment several times. Turns out, if you're the defender, your best bet is to take a backfall while you draw and fire from the ground. It's harder than it looks to run up on someone laying on their back, feet toward you and reach down to stab them while they're kicking you and taking aim at your head. Especially since it's such a large change from the plan the attacker had when he started the charge.