Many soy formulas are also made lactose-free (without a certain type of sugar/carbohydrate in them) that can be useful for children who have lactose-intolerance (not able to digest milk sugar easily).
Now, possible disadvantages... Soy formulas may not be not appropriate for infants born much earlier than expected (very premature) and/or who weigh significantly less than expected at birth because they can't process these nutrients as well.
Also, if your child develops bloody diarrhea and/or other symptoms from cow milk formulas, he/she may require a special type of formula that is much easier to digest.
Contrary to common practice, soy formula does not effectively treat colic, so it is not recommended that parents routinely change formulas without first talking with their baby's doctor, if colic is suspected.
Finally, in terms of using soy formula as a supplement to, or substitute for breast-feeding, the American Academy of Pediatrics recommends breast-feeding as the main nutritional source. However, pediatricians and other health-care professionals understand that breast-feeding is not always possible or desirable for all mothers.
Thus, formula could be used as a supplement to breast-feeding, or as the main nutritional component. A note of caution though: if you want to do a combination of breast- and bottle-feeding with formula, try not to introduce a bottle in the first few weeks of life. We want to make sure the baby gets the hang of it, your breast milk comes in well, and it continues in adequate supply.
Hank Bernstein
Children's Hospital