American Express cards celebrated 50 years in 2008. The card operates in 130 countries worldwide. Moreover, use of AmEx cards grants their users free miles for flying.
Online casinos too have enjoyed the effect of this. Rome Casino reports an increase in players as more American Express card holders appreciate the option offered by the casino and join for that reason, too.
It goes to show that players have their preferences and online casinos who respect those preferences - in this case a certain payment method or other - don't go unnoticed.