There are few reports about Costa from the English-speaking perspective and the recent reviews are pretty negative. Thought I would give a more positive experience. Quick background - German-American couple, a few cruises behind us (transatlantic, Med, Canary Islands) on both US and German cruise lines. Happy to answer questions if you have any.
Very positive - excellent pasta and desserts at MDR, no issue with ordering mulitple starters/pasta course/dessert and skipping mains if not interesting. Very good entertainment and an interesting variety of modern/cheesy/fun/classic genres. Paid for restaurants (burger, hot pot, sushi, pizza and one room service breakfast) were very good and the prices were reasonable. Drinks were generous, coffee was delicous and I had (almost) no incorrect charges. Charged 5euros for something on my last day (after I downloaded my last invoice) so not sure what happened here. Email out now to Costa. This respositioning itinerary was fantastic and the price was reasonable - 14 days on board from Savona, Italy to Kiel, Germany with 4 sea days. If there are any questions about Savona, Tarragona, Cadiz, Lisbon, A Coruna, Le Havre, Zeebrugge, Rotterdam, Skagen, Kiel feel free to ask.
Positive/Interesting - ship was built for the Chinese market so this was interesting to see Italian design via Macau eyes. Curious how Carnival will use this design in the US market. An unique experience even if this led to some slightly negative things (shops were not to my taste but I rarely shop anyway on board), public areas/pools were smaller than normal (found out on behind the scenes tour that the theater on ships in China is for a fee, not free). Staff was overall very friendly, but seemed unsure of themselves if anything was outside the norm. I had no issue with English but our ship was 50% German and the crew was not prepared for that. I actually did public translating in the theater on disemebarkation day because the crew was overwhelmed and Germans are not patient. Ship cleanliness was good but there was a gastrointestinal illness outbreak the last few days (I blame the other guests here vs. the staff/cleaning). Dishes and glasses were generally cleaned away quickly. I lauged at the trip report that mentioned issues with elevator usage. That was definitely a problem, but we took it as entertainment (like watching a clown car...how many people would try to shove into one elevator) vs. a negative. I loved all the different languages and cultures and we also had fun guessing where people were from based on how they acted/drank/ate. Again, free entertainment.
Negative - buffet was overall not good (only ate breafast and snacks here) and you were served in the main buffet, but if you went to the main restaurant for breakfast, there the buffet was a free-for-all. Not sure why the discrepancy. The food was just a bit boring and repetitive although fruit was good and generous. We actually brought breakfast pastries with us from port on the last few days and just used buffet butter and fruit and that made our experience better. The photographers were annoying. You had to be rude to get them to go away. The communication from Costa was definitely lacking in some cases (due to language issues/general Italian way of life?). There were a few things broken in the room that I could not get fixed - drain plug in sink (important because I wear contacts), reading light near bed. I reported to reception/room steward a few times then gave up. Disembarking in Kiel was chaos (translation issues, delays for other guests) - luckily we carried off our own bags, even though this was not given as an official option.
I would definitely book Costa again if the itinerary was interesting. I would plan on some chaos, avoid the buffet and eat all the pasta I could manage.