I have noticed that more and more RC properties have made a change to the "Rate Rules" (click the link of this name in the RC reservation process to see the criteria and benefits of a particular rate code...in this example AXC).

These revised Rate Rules now state that not only must you book through Amex and pay with an Amex, but that the Amex Centurion travel agent must fax confirmation to the hotel prior to the stay to confirm that the guest is entitled to the benefits.

Bottom line: If you like MO and RC get the Black Card. I got double upgraded AFTER a confirmed upgrade at booking at the RC-CP this week (which, btw, firmed its #1 place on my list of city hotels worldwide. The MO down the street could learn a lot from this hotel). I have also saved close to $2,000 at MOs so far this year not including unpublished double upgrades I have been getting from them because of the card. I know it's just a credit card, but it has some power at these hotels.