I personally hold the Centurion card and here are the benefits for the USA w/ airlines:

Complimentary enrollment in Continental Gold Elite, US Airways Gold Dividend Preffered, and Delta Gold Medallion.

Also, free Priority Pass membership. Also, Hyatt Gold Diamond, Hilton Gold, Starwood Platinum Preffered, Priority Club Gold Elite, Avis Presidents Club, Hertz Club Gold.

Also, admittance into Cont. Presidents Club, Delta Crown Club, and Northwest lounges. Annual fee is 2,500 dollars.

Note, there is no pre set spending limit, but your charges are approved on your previous spending, payments, etc. There are numerous other benefits, but those are the "big ones." In all honesty, if you don't travel or entertain, you are not missing out.