I work p/t as a TA. Carnival is a good line but is less expensive than Royal for several reasons.
Their ships simply cannot compare to the Voyager, Freedom and Oasis class ships. However, that said, there is nothing wrong with them. Yes, they have had their "fun ship" reputation, but they are not the old party ships we all remember from years ago. They have worked hard on getting rid of that reputation and making themselves a family friendly line.
The Valor is a nice ship. You simply have to go into the cruise with a good attitude. If you go thinking "this is never going to be as good as Royal and I'm going to get what I paid for" you're going to find things wrong.
In reality, things go wrong on all lines. Just remember you are on vacation, if something is really bothering you, go to the guest services desk and ask for help. Be polite, don't demand and things will generally be fixed. Remember that they are feeding lots of people so you're not eating gourmet food. Yet that chocolate melting cake is possibly one of the best things I've ever eaten.
You've got some great ports (Grand Cayman, Belize, Roatan and Cozumel), you'll have a twister water slide and the outdoor movie screen, and I would recommend you try the steakhouse (for an extra fee).
I think you'll have a great time. Remember, you're getting a good price, food all day, travel to great ports, and a lot of fun available. Just enjoy!