health? No, my health and the health of my dh is just as important as the health of our children. How can we look after them if our bodies are not functioning as well as they can be?
career? Yes
sport? Yes but there are plenty of other physical activities that a mum could do instead of a sport which was interfering with breast feeding
habit? Yes

There is a little debating going on about a comment along the lines of breast is best. I know that research can slant their findings to support any view, full stop. There is no debating that fact. I agree with mad scientist and I would question what cross section of the population was actually involved in the research? I think breast is probably best for your average healthy woman as far as nutrition goes for their babies. Having said that I'd go as far as to say that the majority of woman in the world (yes when you look at the global population I'd say the majority) do not have access to great food and or lifestyle options or choose a less than optimal lifestyle to be producing milk which is nourishing to their babies. In these cases maybe formula is a better choice for them.
I see your point, however science has determined that even when a mother is malnourished she can still produce good quality milk for her baby. In addition when a baby is raised in a household with poor nutrition he/she is more likely to be exposed to disease or poor health so the immune benefits of breastmilk are even more important. Also if a mother is desperately poor and unable to feed herself properly then the introduction of formula is an expense which is likely to tip the family over the edge. 1.5 million children (data from baby milk action) die worldwide due to not being breastfed, and the majority of these children are in poor families where the mother has been convinced her milk is not good enough due to her diet. Sadly the parents cannot afford formula in correct proportions so water it down or mix it with dirty water (as that is all they have access to) and this is where babies die. In poor families with little or no access to health care I would argue that breastfeeding is far more important an issue than it is in our wealthy western society.

Of course mental health is an important issue when determining to continue BFing or not, and issues such as medication and illicit drug use must also be taken into account. So must personal choice, regardless of the reason why. However the research as it stands is IMHO indisputable. Breast is natural, normal, and formula is an inferior product which is a perfectly valid choice but the fact that it comes with an added risk to babies and mothers must be considered in the interests of informed choice.