I think every ship goes through "sewage smell" issues at one time or another. When we were on Radiance of the Seas it was an ongoing problem and they were gradually replacing the plumbing on all the upper decks.
I sailed on Carnival Liberty in June and enjoyed the cruise but I would have to say that the overall food in the dining room and the buffet was not up to what I had experienced on previous Carnival cruises. Also, the buffet desserts, while pretty, were terrible. Tasteless with icings that tasted like they were made with Crisco and not butter or dairy. These issues were not so bad that I wouldn't book it again. I'm actually sailing on Carnival Liberty next March. However, if the food quality has dropped another notch I may have to rethink Carnival.