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I took a Mediterranean cruise on the "Carnival Liberty" last year and was very positively surprised by the product. Carnival's ships are certainly more "Las Vegas"-like than RCI's, but IMHO service and food were better than what I have experienced on RCI plus the standard cabins are considerably larger. RCI's "Voyager"-class ships have a far superior range of "exotic" activities, however.
I wouldn't hesitate to sail with Carnival, again.