This was my family (myself, 18, my brother, 22, and my parents, 50)'s first cruise, and it was with Carnival,on a 4-night/3-day cruise to Freeport and Nassau, and I'm sad to say that I didn't really enjoy the cruise, although I have nothing to compare it with.
Carnival does really live up to its "Fun Ship" moniker, which is one of the reasons I didn't really enjoy it.
-The staterooms: I had expected them to be a lot smaller than they were, which was a nice surprise. The beds were comfy, and the closet was pretty spacious.
-The staff: Everyone on the ship was extremely nice. The cabin stewards introduced themselves, and mine and my mother's (we shared a stateroom), made us cute towel animals and left chocolates. Everyone in the dining areas was really polite, and I thought Stephanie, one of the entertainment coordinator people (she hosted trivia and other such things), was funny and very nice.
-The Freeport excursion: I highly recommend this. We did a kayaking/nature excursion in our first port, Freeport, which was a bus ride to a river, where we kayaked, then a little hiking to a secluded beach, lunch, and then a bus ride back to the port. Ricardo was our guide, and he was AWESOME. Funny, a wealth of knowledge, nice, and he just made the experience really enjoyable. He offered to take pictures of us kayaking with our camera, and they were good ones, too! The only thing that could be improved about this was lunch, but that's it, and it's a minor detail.
-Things to do: I felt like there wasn't much to do unless you were a kid, a party person, or an elderly person.
-The entertainment: The shows, to us, seemed silly, the trivia and things were just eh, and I was overall disappointed with it, especially since family/friends told us we would really enjoy all of the entertainment. The staff did their best to make it fun, though.
In short: I don't think I'd go on another cruise, at least not with Carnival.