Blade craft is increasingly a valid option for those who can't carry a gun because of increasing regulations, although knife legislation is also coming on strong. Knife is also favorable for those who worry about about having gun in the house with kids. But, precautions should be taken with a knife (knives) as one would do with guns, though accidents with an edged weapon is much less likely to be fatal.
Length limits (excluding switch/gravity blades) vary with states. Some have 3", many at 4", and California and Texas at a whopping 5.5". The FAA limit is 4" although airport security staff sometimes implement the rules according to jurisdiction they are in. Serrated blades are a definite no-no. Stupid rule based on appearance rather than actual performance of a blade.
One disagreement with Ayoob's list. Serrated blades are BEST for aggressive cutting. Most rescue knives have serrated edges and rounded points, allowing quick cutting of seat belts and such without danger of puncturing the victim. For best puncturing/stabbing, make sure the point lines up with the wrist and goes where the thumb is pointed in a saber grip. The blade configuration has little bearing unless there is a double edge (which then makes the knife illegal as a dirk or dagger.)
Best type of knife for self-defense is a fixed blade. But most jurisdictions ban these for carry. California and Texas allow fixed blade carry, provided it is carry in the open. (That's why we see one of our favorite authors in this forum, Animal Marc MacYoung, strutting around with open fixed blade carry in his books. Animal lives in California.)
Folders are compromises for a self defense knife. It's slower to deploy because you have to access and unfold. A split second can be all the difference in the world. Secondly, folders are not as strong as fixed blades, with a major weakness presented by the pivot and lock. Increasingly, knife users are noticing that linerlocks -- popular for "tactical" folders" -- can release with with knuckle gripping, a rapping on the blade spine, or wear and tear of extended use. Lockbacks can disengaged or not engage properly because of lint or dirt in the lock notch, and through wear and tear. Both types of folders should be checked periodically with spine taps to determine lock reliability. In a street situation, Murphy's law may kick in, with the blade folding on your fingers because of a rap, torgueing of the blade, white knuckling, or hitting some hard like a buckle, button or bone.
Fortunately, there are three relatively new types of locks out there that have shown to be strong on tough testing. These locks are the integral lock which uses a part of the handle that folds in on the blade tang as the lock; the Axis (mechanical spring) lock by Benchmade; and the rolling lock by Round Eye Knife & Tool (REKAT) that uses some form of mechanical indented plate engagement in the pivot area. Folders with these locks will cost you a bit more money, but we are talking about your fingers if not your life.
Two knife carry. There are differing opinions on this. Some think that the knife that you use for general utility work, day in, day out, is the best because you know the knife intimately. The opposition say that general utility work, such as cutting cardboard, envelops, etc,, will dull the knife and most folks are too lazy to keep a knife sharp (sharpening is a skill unto itself.) These folks argue that one should keep a folder for utility and a second folder strictly for self-defense. The self-defense should be sharp and kept in that condition for contingencies. A dull knife will have a tough time cutting with a denin jacket never mind a leather one. The problem with carrying two knives is perception in the court of law in the aftermath. The good part of two knives is you can use the innocuous looking utility knife and not have people freak. The aggressive bladed and ergonomic handled, tactical folder stays in the pocket. I'm with the two knives school of thought.
Finally, practice, practice, practice. Take the time to slash and puncture hanging targets using sheets of paper and as well as rolled up newspapers (a harder target) to know the capability of your knife intimately. Practice deploying your knife. This is key for a knife, especially a folder. Also if you are of the two knife school, this drill is very important because you normally reach for your utility and not your tactical. During the day, anytime I am in private area, I do one or two quick draws of the tactical from my front pants pocket. This way I am handling my tactical as much as my utlity. I also keep my utliity in a awkward place like a jacket pocket or briefcase so that accessing the utility does not imprint into my body memory like accessing the tactical.
Finally, carrying a knife has made even more conscious about not getting into a needless fight because of the possible fatal and legal consequences. Only on one occaison did the knife come in autopilot, when I was surprised by a lunging dog on a dark street (The thought of a surprised attack and the action of drawing the knife came almost simulataneously.)
As one would with any weapon -- hands, feets, stick, knife or gun -- think deeply about it.