I will start with the preface that we mostly cruise Princess and HAL, that said, last fall we booked a getaway cruise on the Spirit out of Sandy Eggo, for 5 days to Cabo, for us the itinerary and timing was perfect.
Of the 6 cruises we have done on Carnival (our first cruise was on Carnival Tropical back in 1990) I would put the first 5 ahead of our Spirit cruise. The food was the pits, both dining room and buffet, the service was mediocre, with the exception of our cabin steward, he was exceptional, even a decent meal in the steak house (which you pay extra for) was on the low scale of between good and excellent.
But, keep in mind all opinions are subjective, even mine. Maybe it was a bad week, maybe an off day for several of those that I encountered, whatever the reason, but maybe things have gotten better, I just would never book that ship again.
The panini grill station in the buffet is always salvation if nothing else works, they do a good sandwich. If you do breakfast in the buffet go to the station that is doing cook to order for eggs or omelets, then shop around for the sides to make sure they haven't been sitting. I found on our cruise that the did not use water in the steam tables so there was a lot of cool food in some of the stations.
We were on the Spirit 3/09 out of San Diego, we found the ship to be very clean, nice cabins, EXTREMELY comfortable beds and lots to do. A few things that I thought could use some improvement were, the hot tubs were a little cool in temperature, the gym was so-so, food was okay, the casino was FILLED with cigarette smoke every time we had to walk through it. Overall it was a good experience for our family.