izzy, formula fed people as adults probably don't 'look' any different from others. But a lot of less than optimum health is unseen.
Comparing a group of adults who were formula fed with a group of adults who were breastfed you would (on the basis of research) expect to find a large number of health-related differences.
I could list them but you will say I am scare-mongering, and to be honest, I don't think it's helpful to list them in this sort of debate. People get aggressively defensive about it, and because they don't (as an individual) see or feel any effects of these conditions, they choose not to believe them. I do think a mini-course in statistics and what 'risk' and 'odds' mean might help, nevertheless.
However, I can't think why it is remotely controversial to say that what a baby has as his sole source of nutrition in the first months of life is likely to have a long-term effect on his health.
And that what nature has developed over a gazillion years to actually be this sole source of nutrition is likely to give a better long-term outcome than the milk of another species, additionally tweaked with a lot of largely untested additives from other sources to make it mix better/store better/dry better/advertise better.
Why is this worth arguing about?