I'd rate Costa a ZERO if Cruise Critic would let me. Our first morning, I left the dining room in tears after I was yelled at for requesting a table for 2 (I offered to wait for 1 to be cleaned - how silly of me!) Notifying the front desk, they suggested that we not try the dining room again for the remainder of our 2 week cruise! This suggestion was especially ridiculous considering that they do not offer a buffet every night (even on formal night) and that they - surprise! - charge for room service (though the Costa website clearly states that room service is included in the cruise fare!)
Activities were poorly scheduled, with everything being offered at the exact same time, then nothing else for the rest of the day. What kind of activities, you ask? Costa provides "games at your disposal" an hour a day as an activity, charges for a ball to play ping pong or foosball, and closes the gym before breakfast and after dinner. All shows featured practically naked strippers (Seriously!)
The food was terrible. Desserts all tasted exactly the same and menu translations into English resulted in each meal being a mystery (example: I ordered a "calzone" and was brought a plate covered in small shrimp). I worked in the food service industry for several years and was shocked by the repeat food safety violations I saw onboard, with everything from cross contamination to continual use by the staff of a refrigerator that had a large OUT OF ORDER/DO NOT USE! sign on front.
Worst of all, half the ship came down with Norovirus (not exactly a surprise, given that the soap dispensers and hand sanitizers were frequently empty). Costa DENIED ANYTHING WAS WRONG, took no precautions against further spread, and offered no help to sick guests. I was told that we must have gotten food poisoning while off the ship. The only food we'd had for over a week was from Costa! We were so sick that we were completely unable to get off the ship at the port we were most looking forward to, Boston. Sadly, we were lucky. Speaking to other guests, there were many people who were violently ill for the majority of their cruise. These guests agreed that Costa was doing a downright scary job when it came to cleanliness and common decency/empathy in caring for sick passengers.