Here's my take. I tried a 7 day carnival cruise on the Breeze after cruising with NCL, RC, Celebrity. My thoughts are in line with others. I didn't see any unruly people or vomiting on my Carnival cruise and was generally pleased with the people we met. It was a fun cruise.
I hated the carnival buffet, and food was so-so. Much better on other cruise lines. BUT I think Carnival has the better dining room desserts than RC and NCL. The brunch at sea on carnival was great, as was the tea time (with goodies) and lobster night. They don't have the lobster or tea time on NCL. Nothing like Guys Burgers or Blue Iguana on NCL. But buffet is better and there is always something out. Carnival pizza beats everyone else's.
I find both NCL and Carnival to be a bit of a ***** show getting off at ports as far as crowd control. Celebrity was dreamy. We liked cabins on NCL and Celebrity better too, more modern. The shows on NCL are the best of the all.