I seriously doubt it hurts as much as a guy getting hit, but it does hurt a lot. A lot of is has to do with the flesh being up against bone, not like getting hit in the stomach where there's just those unimportant organs.

Sensitive flesh up against bone... yup, definite potential for pain.

ETA: Yeah, a punch to the boob will hurt. A nice firm finger-stab to the boob will hurt a lot too.