I was hestiant to book Carnival due to the reviews, but was on a tight budget. We've sailed Disney before but decided to take a chance on Carnival.
Embarkation/Disembarkation - all went well. Get a passport - you get the short line. :o)
Crew - the crew was great - from room service/housekeeping to dining room/buffets and general staff.
Cabin - floor was wet and the room smelled bad. Housekeeping dried the floor and sprayed the room, which helped. Lots of closet space but very little shelving and hooks. One king bed and two bunks. You can't sit up in bed due to the bunk over the head of the bed. Interior rooms get very dark (except for crack under door) - no dimmer switch or clock in the room. Don't forget to bring your hair dryer! Bathroom OK, rip in flooring over a soft spot. Two other parties sailed with us - one said their room had a big crack in the wall. The other room was next to the elevator and they could hear it "ping" all the time, but got used to it. Both were "sideways" interior rooms for 2 people - better layout in these rooms.
Dining - plentiful and pretty good, but some items were bland. I agree with an earlier review - the chocolate melting cake is great! Lots of fresh fruits and a great variety of salads if you want to eat healthier. Yummy rolls at dinner.
Bubble card - not worth it. The soda isn't fountain drinks - you get the leftover soda from a can used to make a mixed drink at the bar. The kids said the soda was often flat. Save your money and drink the free juices, tea, lemonade, and water.
Entertainment - many times we found ourselves with nothing to do. The shows were only at night and we couldn't make the first one since we had late dining, and usually too tired or just getting out of dinner when the second one started. Staggered times would be better. Lots of presentations to get you to buy, buy, buy. We did a few activities but really found entertainment to be lacking. Magic show was good, towel theater was cute, marriage show was funny, trivia games were fun - but we really wanted some "shows."
Pool - very small saltwater pool, and two hot tubs always overflowing with little kids. Serenity hot tubs were full of drinking adults. Not much fun there either if you don't want to be wasted in the hot tub. If you do, you're all set.
Gym - looked good and stayed pretty busy. Good daily deals offered by the spa.
Circle "C" (12-14 years old) - one small room. My kids liked the Disney youth clubs and were pretty disappointed with the activities for Carnival youth. They went once and never went back.
Cozumel - the dock is the furthest away from the city center, so you need to rent a car or take a cab to get there. We did some shopping right at the port, had a margarita, and headed back to the ship.
Overall, an OK cruise. Not the best we've had, but we tried to just enjoy being on vacation. If we sail Carnival again, we'll try a newer ship and research the activities/shows more before we book.