We have booked 2 carnival cruises in the past using Cruise411 and for those cruises, they were lower than all the other sites I looked at (cruise.com, bestpricecruises.com, travelocity, expedia). For our 3/20/04 cruise on the Miracle, I used cruisecompete, but then found a great deal on skyauction.com. I did not use the auction part and did the "Buy It Now" feature as we wanted a balcony. The cost was at least $100 cheaper than most of the other sites, including the quotes I got from cruisecompete (which were already pretty good). This was true for our cabin as well as for my sister's 6A cabin. I couldn't believe it. Now, I will say I was checking other ships and dates on all of the sites and skyauction did not always have the best price. I think that just reinforces what everyone is saying....leave no stone unturned.
One caveat...skyauction likes to do their customer service via email. (There is no phone # anywhere on their site.) Once they responded to some of my requests re: dinner times, etc., they did give me an 800 I could call with questions. However, if this makes you nervous, you may be better off booking elsewhere.
Good luck!!