I think I summed it up in a sticky I have at the top of this thread...
Costa is basically Carnival for the European cruise market. Carnival is a very good mass market cruise line. If you want to go to Disneyland France then by all means go - but you are basically just getting a French version of something you are already familiar with.
Costa is the Italian version of Carnival. That is what you get. The newer Costa ships are really pretty good, just as the newer Carnival ships are very good.
I think Carnival does a very good job of improving their existing product all the time - while other cruise lines tend to focus more on building new ships instead.
The drawback with Costa is that it can be a tad unfamiliar to Americans. When you are traveling in Europe, sometimes you want to experience Europe all day, but when its time to go to bed you want your CNN, movies, onboard entertainment, etc, to be in English. On Costa they will be in four other languages before they get to English. It isn't HORRIBLE, it is just different. Its very acceptable to Europeans, they are used to it.
But for Americans, if you can find a ship which is your "home" while you are overseas, and that home is American-oriented (your food, language, etc). Then sometimes that makes seeing Europe more enjoyable.
I would not steer people away from Costa, I would just say "understand what you will be getting onboard before you book it").
You have make sure your tour guide will speak English, for example. You have to understand that all the other passengers around you will be speaking a foreign tongue. The entertainment will be largely visual and don't expect any onboard lectures about ports of call in English.
You can certainly go on Costa and have a great cruise if you do your own research on the destinations and have a plan. But you have to be prepared to work a little harder to make sure you find things in English, etc. There will be fewer tour options, fewer TV channels, fewer menu items, that are "American." Some people will find that to be part of the fun, while others will find it confusing and challenging.
I'm not saying American's are too stupid for Costa. I am just saying that sometimes it is easier and more comfortable to be with your own kind.
Even when American kids backpack through Europe they tend to find each other and hang out with each other. Its perfectly normal.