La Leche League will tell you that as long as the baby is latched on right, breast feeding does not hurt. That's bullshit. For the first couple weeks, it hurts like a bitch. First it causes cramps (to prevent uterine hemorrhaging and also to return the uterus to its pre-pregnancy shape and location) and then there's just sore nipples. But after the first 2 weeks or so, it doesn't hurt at all unless the latch is wrong, and sometimes not even then. But if you're just starting and it hurts, and some idiot tells you that means your doing something wrong, ignore them.
Your breasts make colostrum before the baby's even born: yellowish sticky droplets that look nothing like real milk. This is all you'll make for the first 3-7 days of the baby's life and when other moms are shoving 2 ounces of formula into their babies you might feel like your baby can't be getting enough. Wrong. A newborn baby's stomach is literally the size of a marble, and it doesn't stretch. Most of those 2 ounces of formula generally gets spit up, while the colostrum that newborns need gets swallowed and digested and absorbed. Also I have heard mothers say that they had to supplement even before they left the hospital. Who lets them think that's right?
Breast fed babies eat every 1 1/2 to 2 hours. Formula fed babies can go 3 hours between feedings, but not because formula is any better than breast milk. In fact, it's just the opposite. Formula is chemicals, synthetic vitamins and minerals created in labs, and it's hard to digest. For this reason it sits in the stomach longer, making the baby feel full for longer, and causing the baby to skip meals. Breast milk is absolutely ideal and perfect for babies and is digested quickly and efficiently, giving the baby space in her stomach for the next meal to go. Nothing is worse than hearing a mother say that she supplemented because the baby ate too often, except to hear that she "scheduled" feedings every 3 hours or more.
Breast feeding is easy. Formula moms will tell you they used bottles because of the ease, but they're misguided. Formula has to be purchased, measured, mixed, warmed, and the bottles and nipples have to be sterilized. Boobs get washed in the shower (but don't use soap; it can dry the nipples and dry nipples can get sore from nursing) and that's it. No supplies to carry around in the diaper bag (except maybe a blanket if you're shy) and milk is always warm enough, never goes bad, never needs to be mixed or measured, and in the middle of the night you can nurse while you lay down and doze. In the very beginning, and usually only with the first child, it's all about how long on which side, foremilk and hindmilk, proper latch, and remembering which side you nursed on last, but after a month or so it becomes second nature and is the easiest thing in the world.
Breast fed babies have less colic, fewer ear infections, and carry their mothers' immunities longer than formula fed babies. They also test higher in school later on, probably because of the species-specific fats and proteins in breast milk. Also, formula fed babies are more likely to suffer stomach problems in infancy due to the cow-specific fats and proteins in formula. Obesity is more frequent in formula fed babies, too. To put it bluntly, nature intended human babies to drink human milk; cow's milk is for baby cows who are supposed to put on hundreds of pounds right away. We are the only species that, as a common practice, feed our young the milk of another species.
Breast feeding does not cause sagging. Pregnancy does. Breasts grow during pregnancy, and engorgement happens whether you nurse or not, both of which cause stretch marks and sagging. Nursing slows the shrinking back of breast tissue, often giving the supporting muscles and tendons time to adjust. If you just let your milk dry up right away the tendons and muscles can have a hard time keeping up. Also, women who nurse are more likely to wear a bra (often even to bed) in the beginning, helping to prevent sagging.
Nursing burns around 500 calories a day, helping women to lose pregnancy weight faster.
Formula, while generally safe, is always vulnerable to manufacturing errors, product recalls, and bad water. If your city is under a boil order and you don't find out until morning, all those bottles you fed your baby during the night become dangerous. If there's a problem at the formula factory and they recall a billion cans, how much of that formula was already eaten? Breast milk is safer from outside contamination. As for inside contamination, the myth that nursing mothers have to eat the perfect diet and abstain from all alcohol or medication is just wrong. Just like during pregnancy, a nursing mother should take a multi-vitamin and whatever nutrition she doesn't take in will be given to her milk rather than to her; the species is designed to propagate itself even at the expense of the mother. As for alcohol, one drink is metabolized per hour, from the blood and the milk. If you finish your beer at 6:00, you are free to nurse guilt-free at 7:00. And many medicines are considered safe while nursing, including many pain medications, cold and flu remedies, sleep aids, and antibiotics. Even a lot of birth controls are fine to take, depending on how the hormones might affect milk production.