Carnival has undergone a complete replacement of all their beds and reports have been that they are wonderful. I would post this question on the Carnival forum to get a variety of opinions on the the different beds and specifically about the ship you are sailing or planning on sailing.
We had our first experience with the new Carnival beds last month and they were great! I have back problems, too, but too firm a bed makes my hips hurt. These were very firm, with (I think) a pillow top over the mattress...exactly the right combination...for me. Everyone's needs are slightly different, though.
Marty, who wishes I had one right now
Only one cruise under my belt and it was on carnival. Whenever I travel I always pack an air mattress with me so I know I will have a comfortable mattress to sleep on. I have lower back problems and sleeping arrangements during travel is a major issue for me. Sleeping on carnival was a dream come true. They were unbelievably comfortable! it was like sleeping at home. This reason alone would be why I would cruise carnival over any other line.
RCI Majesty of the Seas Aug 2007
Breckenridge ski trip Apr 2007
Carnival Fascination july 2006
On our cruise in May, my husband and I both loved the bed!!! He had back surgery in Jan. 05, and since then, some nights the only rest he can get is on the couch. But we both slept like babies. The new bedding that Carnival has is wonderful, and I thought the bed wasn't too hard or too soft.