We have cruised on Costa twice, in 2010 on the Luminosa, in 2011 on the Costa Deliziosa. I am amazed at the low average score. he last cruise we boarded the ship in Amsterdam. Reasonably fast boarding procedure but it will be faster if you are a Costa club member.
The Costa Deliziosa is a large ship but with more elevators than you can you care for.
Our cabin was nice and spacious and light, the balcony had enough space. You can't book the weather but we spent lots of time outside.