The pain is at most like a mild bruise. Gently massaging the area helps with the discomfort, and is supposed to help with development.
I don't know if that's true or not, but it sure feels good, whatever it does.

Oh yeah, and they'll itch, too. Itchy + soreness = development. Everyone has different times, mine are itchy for a couple/three days, followed by itchy + sore for a couple/three days, and then over the next week the breasts feel larger (or just "more"), then for a couple weeks there's no soreness...then it begins again.