Just finished my first cruise with Costa. Have a lot of impressions to share. First, the best thing about Costa is the price. Because my wife and I have 3 kids, we have to book two rooms. This seems to be standard across the industry (no more than 4 to a room). So with Costa, we could afford to book two rooms for the family.
We were 5 of the 48 Americans on Board (out of 3500 total on board) and we never met another one during the 8 days. So if you're going with the expectation of meeting people or socializing, get cozy with the cabin stewards (who speak English well), or learn Italian. No attempt was made to introduce us to any of the other English speakers.
If you've traveled around Spain, France, and Italy, you know that people don't "queue" like they do in the UK and the US. Generally, people crowd around a counter and let "survival of the fittest" rule. Not a complaint, just the way it is. You have to decide for yourself if you want to be on a ship with 2000 passengers who don't like to wait in line. Also, leave your sense of personal space behind. I move through a buffet line pretty quickly, but I still had people close enough to be sharing my clothes and breathing down my neck. My advice is to get to the buffet at least 30 minutes before the crowd hits, and everyone seems to behave themselves. I wouldn't wish noon at the buffet on anyone.