I've booked a couple of times through a known website and have received the best service and the lowest pricesóat least compared to what Carnival was offering. For my next cruise in Dec. I decided to try booking directly through Carnival but will never do that again--I've had nothing but frustration dealing with them directly. They have always been friendly, but not very efficient.
Some of the travel websites offer promotions like a $50-$100 Master Card credit--it just takes some searching and making sure that you are comparing the exact same items--taxes, port charges, etc.
I saw a post on some board where the fellow claimed that he searched for the lowest possible price on the web and then called Carnival directly to see if they would beat that price. According to him, they always did. I havenít tried this approach so I donít know if it works or not.