My sweetie and I did this cruise for our first vacation together. Overall, the cruise was a great value: we paid only about $900 total for our (interior) room including taxes. The food was very good (though more limited choices compared to Royal Caribbean), and the onboard activities were sufficient but less numerous than what I have seen in my Royal Caribbean cruises. The ports of call are great for this itinerary.
I would definitely do Carnival again, if I found similar rates. However, there were a number of gotchas that you should be ready for:
1. There is only one outlet with one prong in the room we had, so you can only recharge one electrical item at a time. If I had known this, I would have packed a three for one outlet in my bag. Useful to know.
2. If you don't book your excursion with Carnival (and really, you shouldn't, since you can get the same ones over the web for about half the price), there is an issue at Belize. Pandemonium ensued as they distribute tender tickets very haphazardly at one of the lounges for every person that is not on a Carnival excursion, and it is quite possible to miss your tour if you are not someone to bully your way through a mass of people to insist on an early ticket.
3. The Blackjack tournament in the casino is a rip-off. I was only there for two hours of the three hours allocated, and yet I saw a total of 53 registrations (at $20 a pop) during that time. Yet they only paid one person, and only a total of $500. Even if you assume that only 17 were processed during the hour I missed, that would mean they took in $1400 and paid out only $500, for a payout of less than 40% of what they took in. When I inquired about it, the Carnival employee running the tournament out-and-out lied, saying they only had 30 total registrations. Avoid it!