It is not considered as good. Milk still has important components which soy doesn't have. Also, some babies are allergic to soy and it can cause anything from a rash to constant diarrhea (of which the parents are not always aware, because of the form it takes in a baby that feeds on milk (fluids) only).
The very best thing is of course to breast feed. Not only nutritious wise.
In cases where this is not possible the next best thing is milk based formula, and I also heard about goat milk, which is considered very healthy.