We avoided cruising with Carnival for many years because we wrongly believed that it was a step below other lines. Turns out, it's a fabulous experience, just less elegant and with a younger crowd.
Embarkation and debarkation were flawless. Staff was friendly (although not quite as helpful as I've seen before) and the cabin was always spotless and turned down for the night. The food is very good and there is something for everyone, although dinner was not quite as upscale as you will find on Celebrity or Princess. Yes, you get a lobster tail, but that is one tiny lobster. :-) Wait staff is excellent, with the exception of the table cleaners during the breakfast buffet. We had a hard time finding a clean table, which was ridiculous. The staff was extremely accomodating when we asked to change cabins due to engine noise. It took a couple days, but they happily moved us into an upgraded cabin that solved the problem and was much quieter. They couldn't have been nicer when it came to this request.
My only real complaint involved the two Asian singers who sang all over the ship - sometimes in the lobby bar, sometimes near the casino. The first time I heard them, I thought it was really bad karaoke. Then I realized that they were the paid talent, and this just blew me away. These two could not hit a note, didn't know the words to any of the songs, and literally la-la-la'd their way through the massacre of every song I used to like. My tablemates and I made a joke of it - where would they show up next, was the person who hired them thrown overboard? With the other strong talent on the ship, this was the one low point. Carnival needs to rethink their prominent position and time on the ship.
One other note about this ship compared to others - people do not dress as they should for the restaurant, and the dress code is not enforced. This was disappointing and took away from the elegance of the ship.
What can I say about the ports? They are exquisite and each one offers a lovely experience. In St. Lucia, book an excursion with Cosol Tours and enjoy all the food, rum punch, and lengthy tour around the island. In Barbados, I booked a motorboat snorkeling tour with Tristan at Powerboats Barbados. We had him all to ourselves so it was like a custom tour and could not have been better. We saw some of the big operations and there were tons of people all trying to squeeze in with the turtles. I highly recommend this option instead. In St. Kitts and St. Martin we took a taxi to a beach and just enjoyed the beauty. You really can't beat either of those islands when it comes to beaches. Be sure to give your taxi driver a time to pick you up. They will put you with other couples so that you can share the expense of the trip, and it is quite a cheap way to spend the day.