where do you usually aim at with Pi Quan, the neck the throat or somewhere else?

Anywhere there is a target! Pi is a direction of force (downward) not a specific strike. Can be anywhere.
Jake

I like the collar bone or bridge of the nose, but, ya, pretty much where ever they let you hit them with it.

it doesnt really have to be a strike, it can be a trapping technique as well.

the left hand can come down in front of the opp face, and from here, if you are positioned correctly you can block and sweep backwards, or follow through with a throw by going under with the right arm.