At present, there are only Centurion Lounges located in Dallas (DFW), Houston (IAH), Las Vegas (LAS), New York (LGA), Miami (MIA), Seattle (SEA), and San Francisco (SFO).
On 2 Oct 2017, the Amex Centurion Lounge network welcomes Hong Kong as its newest location, and it will be located in Terminal 1. Have a look at these photos courtesy of Amex.
In addition, the Amex Centurion Lounge network will be adding Philadelphia to the list as well before the end of Oct 2017.
Personally, I have been to the Centurion Lounge at Houston (IAH) and I was very impressed by what the lounge had to offer. However, it was a little packed, as Amex used to have a more liberal lounge access policy. Thankfully, according to Amex, from 2 Oct 2017 Centurion Lounge access will strictly be limited to Platinum Card and Centurion Card members only. All other Amex card members will no longer be granted access to Centurion Lounges, as per the statement from Amex below.
This is good news for travellers who wish to find some peace and quiet while waiting for a flight.