I was a formula fed baby as I was very premature and didn't have the suck reflex. Back then, they just put you on a feed and didn't ever try to go back to breast feeding. I turned out fine - if I do say so myself.
That being said, I will be trying my darnedest to breastfeed. As an amateur nutritionist and professional chemist, I know that there are antioxidants and micronutrients (vitamins) that we haven't even discovered yet. However these antioxidants and micronutrients exist in our food and our breast milk. They are not replicated in formula b/c we are simply unaware of all of the nutrient properties of breast milk. So, just like we can't simply take a multivitamin and then eat only junk food and be healthy, I think formula is sort of like a vitamin that does a good job, but not the entire job - YKWIM?
I will be going back to work after 6 weeks, so I will be pumping and letting daddy do the late night feeds! I think 4-5 weeks is the best time to introduce a breast fed baby to a bottle (to avoid nipple confusion).
So I think breast is best, but if there is a medical reason to formula feed - don't beat yourself up for it. Loads of babies are perfectly healthy on formula.