In an ideal world breast is obviously the best thing for a baby but as we all know the world isn't ideal. BF is brilliant when it works, when mum has enough milk, when baby latches well, when the let down isn't painful, when there is no mastitis or nipple damage and when mother and baby are both happy and relaxed and have sufficient clinical and pastoral support to help if things aren't working as well as they could. The bit they don't tell you at ante natal classes (and it's a bit like the perfect birth) it doesn't always happen like that. Many many years ago there was an alternative - a wet nurse whose baby had died due to illness or a traumatic birth and the lack of medical care then available. Formula is a brilliant alternative especically in the Western world where there is an adequate supply of clean water. It was invented because there used to be no alternative and mothers and babies suffered terribly as a result. In an age when antibiotics were not available mastitis for example was a killer.
Of course breast is best but I think we sometimes forget that it isn't the be all and the end all and when it goes wrong the fact that we can turn to formula is a blessing. A mantra has developed that breast milk has mystical and magical qualities not disclaiming the antibody factor here, but at the end of the day formula can be a life saver both physically and metaphorically.
I BF ds until he was 8 weeks old. Infective mastitis (thrush of breast - realised that years later when I read into a bit more) a breast abscess and for two weeks because of the continuing mantra tried to carry on with a tube draining the abscess taped over my back. During this period my HV actually told me that bf mothers put their babies first and FF mothers put themselves first!! Eventually an NCT expressing machine lady took one look at me and went and bought me a set of bottles and a tin of formula and showed me what to do. I feel forever graeful to her but it didn't prevent three months of clinical post natal depression. Looking back I'm still not sure that the benefits of breastfeeding during this period outweighed the three courses of antibiotics and painkillers I was taking 
I managed to feed dd for nine months - having read a great deal in the meantime and having engaged a bf counsellor from day one to help. The guilt that was imposed in relation to ds though even with hindsight was wholly unforgivable and there is never, in my opinion, ever a reason for any woman to suffer because breast feeding isn't working out.
My DC both had bronchiolitis, both had wheezy spells and a touch of eczema DS more chesty and DD more exzemaish, both had rank persistent ear infections and grommets before the age of two. Both now big, strong and very bright: 15 and 11 respectively and you know what neither of them remember BF or FF because it is a miniscule part of being a mother.
Sorry - bit long.