Sometimes they are great. Sometimes they are not. There is no way they can always be great, but it almost never hurts to try and ask the Concierge for help. But, Amex should know that customer service is all about expectation setting.

IMHO, Centurion could do a better job setting expectations of what the Concierges can and can't do, and what they are more willing and less willing to do as well as the goals of the concierges.

For example, I have had trouble finding the following:
a) Does the quality of Concierge service vary depending on your charge volume or type of charges you make?
b) If you make repeated requests of the Concierge, but then don't make purchases resulting in extra income for the concierge department, does that make them less willing to help you in the future?

Maybe they should also do email-based surveys to monitor customer satisfaction for each request.