I've used two credit card concierge services, but both are in Australia. One is the Platinum Visa, which was marginally acceptable at best, they will do simple requests, but anything complicated, or which requires them to not pay the highest price on the planet, they are hopeless.

The second was AXA Assistance, which came with my bank account. It was better, and they actually make an effort for most things, the only thing is they are not at all what you would call "exclusive", so they dont really get any special treatment or offers, but good for find out information, and the like.

In Australia Amex Platinum is still the best.