I had planned to breastfeed, but my twins were born at 34 weeks and ended up in the NICU. Their doctor started them on formula before my milk even came in (without asking me, though at that point I probably would have done anything he recommended because I was scared). By the time they came home, they still had difficulty even taking a bottle, and never did learn to latch on to breastfeed. I pumped for 2 1/2 months until my milk supply dried up, and mixed the milk with formula so that I had enough to feed them.
The lactation consultant at the hospital wasn't much help.
If you have already nursed one child, though, you already know what to expect, and will probably do a lot better than I did. I'm going to guess that a lot of the women who are supplementing either (a) have preemies, or (b) are first-time moms who have never breastfed a baby before.