I know I'm late coming into this one but for some reason I just can't let this one sit.
First of all, I agree with Greg again. (That makes twice today)
Defense by nature is always LATE! Most victims of voilence are covered with "defense wounds" at their autopsy.
If you are moving first, at least you know when the attack is coming. IMO your psyche should always be set in attack mode. This does not have to be violent. It just means that your intent should be to take over your opponent's/partner's center...even with just a touch. My teacher says if you don't want your partner to touch you touch him first.
Second, the truth is, you have to be vulnerable to be dangerous. This is the nature of our practice. What was almost a good gyaku game ate can become tai otoshi in the blink of an eye...and vice versa. Giving up to this vulnerability during training to develop good solid timing and technique is difficult enough with people you trust much less with a thug that wants to cut your head off as you walk out the bar with his girlfriend. Eventually you realize that outside of the dojo, most people don't know what to do with your center when you hand it to them! But then again, most people are too afraid to do it.
Finally, yes, Aikido is a very effective form of "self defense". Or should I say self preservation. But not because of the superiority of the techniques. It's because of the principles that get instilled in your mind and body by practicing the techniques. Also, training fosters a certain awareness about other's intent that is hard to develop anywhere else. For some reason, which I don't fully understand yet, it is very difficult, if not impossible to hide your insides when you are sticking your hands in someone's face. Picking up on these "vibes" during class translates to feeling and seeing specific danger and malice toward you in the world.... and goodwill for that matter. Again, choosing to listen to the cues and act is another story. Learning to trust your intuition is the key. For me, seeing deceitful intent is easy. Deciding what to do when I recognize it has been very difficult. Probably cost me a job or two and several relationships. You win some you lose some...but you have to do SOMETHING!
P.S. I'll never reply to an "Is Aikido good for self defense" post ever again.....not here anyway