Amex is accepted almost everywhere online, anywhere Square/Revel/Clover is used, any large retailer, and most 2+-dollar-signs-on-Yelp restaurants. As you said,

Amex is accepted almost everywhere online, anywhere Square/Revel/Clover is used, any large retailer, and most 2+-dollar-signs-on-Yelp restaurants. As you said, users should still carry a backup, but most places accept it. As for customer service etc., their platinum cards are where they shine.