I know this is slightly off topic, but it is the reason I will always have an Amex:
I was in Barcelona for two weeks right after Amex began to issue cards with chips, and as most of you know the cards with chips cannot be swiped. I'm not certain whether this is still the case, but at the time that is the way the system worked. Either my Plat Amex chip never worked, or was damaged at some point (this was the first chance I ever had to use the chip function). At the time, it was the only card in my wallet that didn't charge an fx fee. Finally, at one merchant, I figured out the card wasn't declined, but that the card reader said the chip was bad. I called Amex that afternoon, in Spain, and I had a brand new Amex card the next day around 2PM Spanish time. That was the best customer service, and in stark contrast to two situations I've had since with Citi. On both occasions, Citi refused to send me a new card to an international destination.
Not specifically related to the concierge service, but it's the reason I generally use my Amex first, and one man's opinion.