From what I understand, the high doses of vitamin k can be a shock to the newborn's system. Also, it only matters for a few days. Babies are born with a good deal of Vitamin K. It just takes a few days for the baby to start building up vitamin K from breastmilk. I would consider it if our baby had clotting risk factors or if we had a son who we were thinking of getting circumcised, but if we have a boy, we are not going to have him circumcised. Even if you are planning on circumcising your son, you may still avoid needing the vitamin K shot by postponing the circumcision at least a week.
I have heard that the oral vitamin K is less of a shock to a newborn's system, and I would consider it except that I would like to attempt to maintain pristine gut. Basically, the principle is that as long as breastmilk is all your child is eating, your child's digestive system is coated to help provide a barrier against germs and allergens. I'm just not sure I want to mess with that on day one unless there's a really good reason, and I don't see vitamin K as a good enough reason unless there are risk factors.
Basically, I understand that newborns may have more difficulty with blood clotting in the first week or so of life, and I think that warrants taking some extra precautions but not an invasive shot or oral supplementation for children with no other risk factors. Personally, I think the most important think I can do is try to protect my child from injury, especially head bonks within the first few days of life. The second thing I plan to do is increase my vitamin K intake just before and just after my baby is born in hopes that there may be a bit more vitamin K in my breastmilk at first. Here's the USDA's list of foods containing Vitamin K: http://www.nal.usda.gov/fnic/foodcom...k/sr16w430.pdf