The Platinum Amex charge card has an annual fee roughly equivalent to purchasing an airline club membership. You must be flying that airline to get in. Also it includes Delta, Continental, Northwest, American, maybe some others. If you are flying United through ORD you can't use any lounge since UA does not participate. You would not be able to take your UA ticket over to AA to get in their club.

it's definitely a good perk to the FEW (relative to the airport as a whole) people who do have the Platinum AMEX charge card.