Its always about shot placement anyway. .22 will kill you, just ask RFK. You get a good hit in the "T" or the brain, or brain stem, you're going down.
Not to mention there are major arteries that can be hit and you can bleed out fairly quickly.
Sure there probably isn't a huge amount of knock down power, but if you can really shoot and hit the target where you aim, and also, know where to aim, then a .22 is fine.
I don't carry a .22 for self-defense, but if its all I had, I would.