The way I look at it my primary carry gun is a BUG -- it is my last resort. I might feel differently if I wasn't a home body, lived in a different area, or found myself in risky situations more often. There is no doubt that they can and do save lives.
After exploring the idea myself and doing some reading, I've personally think a knife is a poor choice for self defense. It's not because it's not effective. It can be real effective in the hands of someone who knows how to use it... even at the ranges associated with a typical gun fight.
The problem is the knife is the hallmark of the bad guy. Think about this hypothetical situation: You are in a convenience store. The guy at the counter pulls a gun on the clerk. He looks at you and tells you to stay where you are or he'll kill you. You feel like he's going to kill you and the clerk anyway. Wanting to stop the threat, you pull your carry knife, sneak up behind him, and slice his carotid artery He remains a threat for about 1.5 seconds and then he's done. Now think how that might land on a jury... I believe the guy with a knife who knows how to use it is going to jail... almost every time -- that's a scary dude that people just can't relate to. I feel like the person who used a gun to stop the threat would have a much better defense in court.
I don't have much to base my theory of knives on, so I could easily be wrong. It's just the way I currently perceive things.