However, I have been present for numerous baby blessings, including my brother, a few nieces and nephews, and children of close friends. I have witnessed the power of the priesthood as the brothers stand in a circle, holding the baby. Words said in the blessings are different, and comes from what the brother giving the blessing feels impressed by the spirit to day. Blessings can be short, medium, or long. I've heard some refer to the baby's eventual choice of spouse. Some talking about how the baby will affect others in its life.
Blessings are usually administered by the baby's father or another male relative. (My dad has blessed one of his grandsons.) In some cases, the priesthood holder blessing the baby is of no relation, though I haven't been present for one of those, yet. But I was present for mine. :-) And it was my father who gave it to me. While I think it would be nice to know what he said in my baby blessing, it isn't necessary. I know that my father spoke words of love, guidance, and protection that my Heavenly Father wanted said--and I believe that as I try to live righteously every day, that blessing surely comes to fruition every day.
More specifics about baby blessings can be found here.