With my first child, I only used enfamil. I bought it at Costco too because it was cheaper. As a new mom I felt there was no way I was using generic. But with my second child, I did use the Costco generic brand. It is a lot cheaper, and I really don't think there is that much difference. My sister in law is a nurse and she used generic with both her children. My son is in the 95th percentile for his height and 90th for his weight, (he is now 2 1/2 and not chubby). Very happy and healthy, he's only had two ear infections his whole life. Don't sweat it, kids drink formula, generic or otherwise, and grow up healthy. (I do commend you for breastfeeding, because I think breast is always best, but I stopped nursing after 5 months with both kids, and they turned out great)! I hope this helps ease your mind.