I took a cruise on this ship in July in the Mediterranean with a group of 19 (all family). We had a great time.
Although it was different from the typical US cruise, the difference made it interesting. English is not the only (or even the main) language. But that was not a problem (I speak only English). Each "language" has its own "host" who meets with you. Group activities are done mutilungually, with English being one of the languages.
It was fun being immersed in different cultures. Although a few people were rude (cutting in line, etc.) most were not.
Our ship we full, but it never felt overcrowded. They have plenty of space to accomodate everyone.
One thing that was a slight negative was that the cabins are a little smaller than the other ships I've been on. We knoew that going in however, so it was not a surprise.
Overall, I give the Costa Concordia high marks and would do the trip again if I had the chance.