Why should you say anything? She is a big girl and can have her own opinion on breast feeding and doesn't need to be persuaded by someone who had a different experience with it than she did. All the talking in the world is not going to change her thoughts on the subject.
I think that the value breast feeding has is an opinion that is held by each person, and each person should be allowed to have their opinion in peace, without getting beat up by the other side. In most situations it is the best for the child and mother. However, there are always exceptions to any rule. And just because something is great for you, doesn't mean that everyone will have that same great experience, nor will you be able to dictate how someone feels about it.
In my case, I breastfed my first three children and struggled with each one. My fourth child who is now 2 months old, I tried it with and just couldn't do it. I could have continued to stress myself and her out, plus starve her, to keep on trying, or I could say..... thank heavens that while formula is not breast milk, it is a great alternative for those who can not breastfeed their child for whatever reason. And yet because of the influence of society, I still felt guilty and like I should explain myself to other people, how insane!
I think that it is sad that we live in a society where people's opinions on a matter are used to batter other people. Her experience is her experience, and no amount of your telling her how wonderful your experience was and how beneficial it was will change her mind. Probably just make her think less of you.