We did the christmas cruise on the Costa Serena in December 2007. Summarizing, we had a very nice trip, enjoyed most of the ports and the new ship. There are some BUT's I would like you to observe, if you don't mind them, you will have a nice cruise.
Overall, until now we did cruises only on american ships, on NCL and Royal Caribbean, so we tended to compare Costa with them.
First of all, we travelled with my parents, booking two balcony cabins 8383 and 8385. Cabins are perfect (no wonder, the ship is nearly new) and spacious, enough space. I really advise to book balcony cabins. Some people say its a waste of money, we don't want to miss the balcony, its perfect top sleep with open doors and enjoy sunset on the balcony early morning.