Have been a frequent cruiser on Carnival and usually have had very good experiences. Carnival is not, nor does it pretend to be, an upscale or "luxury" line. You're on the fun ships, after all. What you will get is decent food, great drinks and pretty good service. Excursions tend towards the beach and water-minded, as well as the party crowd and more educational outings are in rather short supply.
That said, here is the good stuff.
The aforementioned food has improved dramatically and you can get a good meal whichever venue you choose.
Great tour guide, but hard sell in the shops and town was so dirty. Boring...
We missed the tour we really wanted, because the information person on the pier mistakenly told us it had already left. There were 2 options for that tour - 8:30 and 9:00 and we had the 9:00 tickets. He apparently didn't know that there was another one. By the time we got it all straightened out (the gentleman at guest services apologized profusely and immediately processed a full refund. We wanted to do something, so we purchased tickets for the only tour left - the mixology class and salsa dance class. Thought that would at least be fun. Just okay, nothing special at all.
We aren't beach people, so we just walked around and had drinks. Really picturesque little place and we had a good time just being together:)