I sailed on the Concordia in January of 2007 and it was fantastic. The crew are polite and courteous, the food is good. The main dining room is the best, the breakfast and lunch are just as elaborate an affair as dinner.
There is no self-service laundry aboard the ship, take your own laundry detergent, so that you can at least wash underwear and socks in the sink. Take extra clothes, so that you hopefully won't have to use the laundry service. Remember that this is in Europe, so EVERYTHING is charged in Euros. My husband and I sucked it up and paid to have the laundry that we needed done, and I attempted to wash the rest in the sink. NOT SO PLEASANT!!! I really recommend taking extra clothes just in case.
Other than that, we had a fantastic time. I would book on the Concordia again, having gone on a Carnival cruise two weeks ago and not liking it.