I can't comment on their interest rate since I never carry a balance. If you plan to cruise with Carnival anyway it might be worth it. It's pretty rare for a CC company to pay you $1200 for using them for 18 months. That said, I don't recommend them. Their customer service is not good at all and you are "locked" into Carnival. As per my post, they obvious don't outline their rewards programs very well.
Use them if you know you want to sail Carnival and you understand the system. As for us, it's becoming our secondary card with Discover taking over as primary.