I've been on Carnival, RCCL and Celebrity. I agree with everything that Dave said. I've found the food on Carnival better than RCCL, and RCCL better than Celebrity.
On a lot of the Carnival cruises, the waiters did sing and dance every night after dinner. I didn't like that at all. However, on our last cruise on the Victory, they only sang one or two nights.
Dave's right about the Bingo and the announcements on RCCL and CCL. The one thing I liked about Celebrity was the lack of annoying announcements. The most annoying anouncements are the ones given by annoying CDs. If I like the Cruise Director, then the announcements don't bother me quite as much.
The cabins on Carnival seem to be a bit a larger to me than the ones on RCCL. We always get a balcony, so that's whay I'm comparing. They aren't huge by any means, but they are a bit bigger. And, the beds are more comfortable. Carnival updated their beds, I believe, and the last time I was on RCCL, they hadn't done it yet.
I think you'll enjoy CCL a lot.
I hope this helps.
Read and post cruise reviews
Veracruz - '85
CCL Tropicale - '94
RCCL Adventure of the Seas - '05
CCL Valor - '06
CCL Victory - '06
Celebrity Century - '07
CCL Triumph - '07
CCL Victory - '08
NCL Gem - '10