I recently took a five day cruise (stopping in Grand Cayman and Cozumel) with two girlfriends. We are all first time cruisers. We went with the Carnival as the dates and port worked well with our travel plans. Overall we were very happy with Carnival. The price we paid was fantastic value. We choose an inside room with three twin beds. we found we had plenty of room. There was lots of storage and the washroom was a decent size. Given the amount of time we spent in our room we were happy with what we had. Housekeeping was fantastic (better than most hotels I have stayed at). We choose the flexible dining plan which worked well for us. The food was incredible. We were thrilled with the wait staff and the overall dining experience. Drinks were reasonably priced and I liked the fact that you could order a bottle of wine and not to worry about finishing it all in one sitting (the friends I travelled with are not wine drinkers).
We did find the daily music/trivia games on the lido deck to be a bit much. We tried to retreat to the "adults only" section of the ship, but it was quite small and we were never able to find available loungers. We ended up spending most of our time on the deck which had the children's' slides as it was not as busy and fairly quiet.
If you want to "party" then you will enjoy Carnival. There is always lots going so you have the option of retreating to your room early or staying up and enjoying the entertainment. The dancers were great as were the comedians we saw. The staff really do a wonderful job of trying to keep people happy.