I went on a Carnival cruise for my senior trip. We had a blast--we thought the food was great, the activities (which I can't remember now) were fun, the staff were friendly, and the ports were awesome.
I say this in full recognition that, as college students, we were willing to eat anything, could make the most of boring activities, and weren't used to being waited on at all.
I am not sure how much of this was attributed to the awesomeness of Carnival or just the fact we were a week away from graduation and no one could bring down that high. But honestly, the other people on the ship were all fun and friendly (probably because they were drunk most of the trip) and the boat had a mini golf course, water slides, and other fun things to do. We were also quite a large group and they accommodated us very well during our special activities together. I would definitely use them again but I've been dying to try a Disney cruise.