I am quite aware that a knife is a lethal weapon, used properly an attacker won't see, they will feel it. Sometimes, however it makes for a good distraction.
As for Aikido, its a good jumping point for dipping into Martial Arts, I have seen it used effectively against multiple attackers. The key is to allow your escape, in some situations that is the only safe option.
Guns, can be effective, you also have to the weigh the risks/liabilities of the situation and above all, be willing to carry out your threat. Shooting a gun at a range, and pointing it at a living breathing person (regardless of the threat they pose) are two separate issues.
Finally my point is, and I maintain, for most people its a liability issue. If an Unarmed mugger demands your purse, and you pull a gun, in my state you would be Arrested. The mugger would have every right to sue you.
Unless there is a direct threat, or a real fear (i.e. work in the bad part of town), carrying concealed, is kinda dumb. Whens the last time any of you had a gun put to there face? I've put myself in those situations, a gun on my end would just have escalated the whole mess.
As for the poster with the .45 comment. If a perp has a weapon drawn on me, and mines in my holster concealed, what are the chances he's going to let me pull, sight and fire? Unlikely.