I've had a Diners Club card with concierge service for a couple of years now, and have used the service only once. Based on talking to people who work in a concierge company they can in most cases only do the things you could've done with a little Googling yourself.

It is nice to have somebody look for flights, hotels or restaurants for you, but in general by doing it yourself you get it done a lot more quickly. 5 minutes to findband book a hotel using the phone browser, vs. about the same time on the phone, and waiting them 12 hours to get it done...