I have cruised twice on Carnival. I don't plan on cruising on them again. I'm not saying that it's bad, it's just that I much prefer Celebrity as far as food and service.
Now Royal has the best ships out there. As far as excitement, entertainment and fun they are a cut above.
Carnival Liberty New Year's Eve 2007
Liberty of the Seas 5/2008+11/2009
Allure 1/16/ 2011
Vision OTS 2-14-2015
Freedom OTS 3/6/16