I just booked a cruise on NCL Jewel. We cruised for the first time last winter on Carnival Legend. I posted a similar question on another message board and the main thing I got was that they are very comparable lines but most people said the food in the dining room on Carnival was better than on NCL but NCL's buffet was better than CCL. Also NCL's entertainment was better. So....because I just want to try a different line and the fare was a slight bit less on NCL for the same sailing/comparable ports, I decided to try NCL.
We had no problems whatsoever with our CCL cruise. We were able to fit 4 (2 adults, a teenager and a child) in a balcony cabin quite comfortably on CCL but we had to get 2 cabins on NCL - pictures I saw with the pull out sofa and bunks made up were enough for me to realize that there is no way 4 people would fit in a NCL balcony cabin! The entire space was covered in beds and the balcony was not accessible with the bunk down.