I'll split my comments in a few subjects in order to make it easier for the reader to get what was OK and what wasn't.
Good things first. The waiters serving our table during the dinners, Paulo and Mario, were just fantastic. They were quite fast, solicit, kind and always prone to help. We were in a table with people from Portugal, and since our native language is supposed to be Portuguese they really tried to speak it - with some success, I must say.
Itinerary was another good thing about the cruise. The cities selected for visiting were really awesome, despite spending more time in Santorini and Katakolon would be required. The Portuguese speaking hostess, Fabiana, did all she could for making our stay more comfortable. Since we were a small group she managed to disembark us fairly quickly so we could get the most out of the cities' tours. She also provided us with tips and tricks for visiting the cities without being bothered by the huge tourists groups, what gave us more time to appreciate what Greece had to offer - even because excursions were extremely expensive and not recommended by people that already did it in previous cruises.
Now, the not-so-good things. Life on board was kind of complicated... Not every crew member spoke English, so we had some difficulties communicating for buying tickets. Costa was supposed to sell tickets for the bus to take us from the Mikonos Port to Little Venice, the selling happening at the Excursions Desk OR at the bar in Deck 5. We went to Deck 5 but as the waiters didn't speak good English it took us about ½ an hour to finally get the tickets. We tried to communicate with different waiters and attendants, but it was in vain. A lousy waiter kept speaking Spanish with me even after I told her I don't speak any Spanish (despite I can understand it fairly well, so I got the names she's called me).
Food is really lousy, cheap, and the menu is not varied at all. I've read several reviews in this website already addressing this issue, but I think it's important to emphasize it again so the readers can make their own statistics on the subject. Meat was cheap and not available. Fruits were not available, something that I and my wife really missed in the 50ºC summer of Greece.
Most Italian passengers were really impolite - crowding lifts without need; they don't really talk to each other, they shout; taking food with their (almost always) unwashed hands (really disgusting!); and absolutely not respecting any line. Crew, attendants and waiters didn't do anything to avoid this, even though they've seen it many times. Notice that I do love Italian people (even have relatives in Milan), but I was really scared seeing so many disgusting attitudes like the ones I've described.
Oh, sleeping was a problem as well, since the cabin was quite noisy. I kept waking up the whole night long for trying to find the vibration points and put some folded paper between the parts. At the end, I was beaten since the more points I located, the more appeared! I've put over 15 pieces of paper on day 1, and found about 20 other pieces of paper put by other people. I guess someone is still struggling to sleep in cabin 2172!
Last but not least - and that's really important if you're still thinking to go to Greece in this ship - life onboard was really, really boring! I've read in Costa's general review that one of their main differentials is the entertainment. This is a lie! In fact the ship has several pubs and bars, but most of them have no attractions during most of the time. Those that have something have the same shows every day. There are no simultaneous attractions in their repertoire as well - if you don't want to make your own "gala roses", you'd better have a set of cards to play, or a good book to read, since swimming pools are always crowded (and usually dirty) and the Casino is really a joke! I've written all of this to the Cruise Manager, to the Entertainment Manager and to the Portuguese Speaking Hostess in a formal letter, but got no answer at all. Therefore, I must suppose they are happy with the current status and, according to the reviews I've read in this website not much has changed! For me, this really shows Costa doesn't care at all for the passenger's opinion and that's one of the reasons why I won't ever get a Costa Cruise again.