CostaMediterranea is the sister-ship of CostaAtlantica. I've been on the Atlantica about 9 months ago - and it was more or less awful (lousy service, lousy food - but great ports of call in the Med).
So I booked the Mediterranea with mixed feelings - but I really wanted to visit its two main ports of call Istanbul and Izmir (Ephesos). Already some 5 or 6 hours on the ship I noticed that this wasn't comparable with CostaAtlantica. The staff was friendly and seemed to love their jobs. The food also was way better than on the sister-ship. I really did not expect that the standards from one sister-ship to the other can range from "stay away" to "great".
Of course this is no 5 stars ship - but considering its official level and pricing you will get very much fun and quality for your bucks.