Just the other day there was an episode of the TV show "The Best Defense" broadcast in which drills were described and demonstrated using .22LR in a self-defense role. They advocated extensive practice and focusing on rapid multiple head-shots, coupled with the use of moving targets to simulate head-bobs and ducks of an assailant. They were using a .22 semi-auto, but it seems to me that the same principles would hold true for a .22 revolver. As others have said, I'd not choose the .22 as a primary SD gun, but in a pinch I'd rather have one than nothing by far, and I suspect that in the hands of a well-trained shooter using proper technique it could be very effective.