Took the 5-day cruise to Cozumel from Galveston in July with my mom and sister. We had a good time together, which is always nice, and our day at Nachi Cocom Beach Club was lovely (though that is not the beach excursion you can book through Carnival so they get no credit for that).
I have cruised with Princess and RC and the Carnival experience was the worst by far.
Our bathroom reeked of sewage from the first day. The food was tasteless and cold; even the desserts were completely uninspiring. There was no wait service in the Lido buffet area, so while employees did come by and remove dirty plates, every time I wanted a soda (or refill) I had to go to a bar and wait in line while my food got even colder. The decor is outdated, but I could forgive that if it were at least cleaned, but there was trash all overin the deck chairs, in the elevators and stairwells, in the public bathrooms, on the decks. Just garbage that people threw down (or, I suppose, that blew from beneath chairs or from balconies). Part of the problem was the clientelle. There's no inoffensive way to state this, and no way to avoid sounding like a snob, but there were some trashy people on that cruise. As a disclaimer, please let me say that I am a Texan myself; these are my people. However, I got very tired of hearing shrill voices hollering "that don't taste good" and such at their children, all around me. I was behind some men at the buffet one day who couldn't discern the meaning of "giblet gravy" (one pronounced it with a hard "g"). I sound like a jerk, I know, but there it is.