I have a USAA branded AMEX which seemed unusual to me, but USAA is an
unusual company. While discussing the way VISA/MC works (with various
lenders setting their own fees, etc) I said that AMEX doesn't have a network
like Visa except for USAA.

Then 2/3 of the people around me pulled out their Costco Brand AMEX cards.

Does anyone know what the deal is with Amex nowadays? Did they switch to a
Visa model at some point? Or is it truly rare?