We used it for our daughter and she never had a problem and now we're using it for our son and he's doing great as well. If you look on the back of the Kirkland package, it has the same ingredients and the same amount of each ingredient as the Similac and Enfamil. Costco is also great with returns so if there is a problem, return it. I've seen people return a grill they've used all summer. $20.00 in formula isn't going to hurt them.
Re: Anyone use Kirkland brand formula (Costco)?
We love it! No complaints here, especially 2 cans for $20. We pay $50 a month for formula.
ETA: It's also foamy, but DS has never had trouble with it and he's been on it from the beginning. He barely spits-up.