I've only ever aimed for the head once where I could've hit elsewhere. If I have a choice, arm or midchest. I play a lot of CQB, so I use full face - and I've needed it - my dye is all scratched from pellets I've taken.
Now, for that one time. I will say I am not really proud of it, but I would most likely do it again. It happened in May of last year. I was training for my shooting class in June in my back yard with my WE g17. As I was taking a break and resetting my targets, I heard a much louder shot followed by my dog (who is my baby) yelping. I quickly round the corner of my house to see a kid with a clear Airsoft gun aiming at my dog, who was cowering on my doorstep. The kid's two mates were also with him. Well, I know what has happened, and I am seconds away from going completely crazy at this point, and I'm too far away to stop them from taking another shot. So, I do what I can; raise my glock, and put two into the side of the kid's head. He yells, drops the gun, and runs off with his mates. Turns out he was using a Cybergun Jericho, which is CO2 and shoots about 400ish with .12g, which he was using... I ended up keeping that gun for awhile, and selling it to a friend to use.
Now, this kids mom then comes back about 1/2 hour later and stars going crazy at me. That is, until I tell her what happened, and show her where my dog is bleeding on her flank. Turns out she didn't even know her kid had an Airsoft gun. Needless to say, I think she took care of the issue - and she told me to keep the gun as well...