When I was in the 7th grade playing little league, I was on 3rd base and got the suicide squeeze sign. The batter missed the sign and swung away, lining one into my cup at a distance of about 20 feet. It hurt like hell and the cup broke into three pieces, but my two kids are proof that it worked.
In my high school career, I lead the league in hit-by-pitch for four years. It does not hurt too much if you can get it to hit you in just the right spot on your back.
The most pain I experienced from a baseball was, while playing catcher my senior year, I got hit in the meaty part of my ankle by a knuckleball that was suppose to be a curve (and the next pitch was fouled off the same spot!).
MotionMan