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!