I have done it a few times when I was younger. The first few times I didn't feel bad and thought it was funny.
When I got older and realised how much it hurt them, I felt guilty and didn't do it again.
I did do it last year but that was when a guy friend of a friend grabbed my breast. I didn't feel bad about that at all!
It is kind of funny when you're young, but it is really not cool. Unless you're being attacked, then try to knock them off!
It doesnt' hurt as much when you are being hit, but afterwords there is a painfull ache that lasts forever. Also you can't walk properly upright afterworde, at least with me. I've been kicked once, but it was by a guy because of a truth or dare game.