I have to say they sure make it easy to find good last minute deals.
However, if you dig a little deeper, you'll find that for the most part most companies are advertising the same prices as the what the cruiselines are. The only difference is that some agencies will bundle certain spiffs for booking to entice you to book with them, such as OBC, prepaid tips, etc... Most cruiselines prohibit agencies from advertising rates below what the cruiseline is offering.
That is not to say that you can't get a better deal, but generally you will have to initiate contact with the agency to get a truly better deal than what is being advertised. You need to email or phone them to get the better pricing, as they are prohibited from advertising it online or elsewhere. Also what happens is the agent is actually rebating a portion of their commission to you to give you that discount.
Royal Caribbean and Celebrity prohibit this, as a result you should find uniform pricing across the board on their product. However, Agencies do still offer certain spiffs for booking, but no discounts.
I have an agent that I work with on a consistent basis, she handles many of the CruiseMates Group Cruises and she gives me a great deal and terrific service. She works hard to ensure my issues are addressed and if I ask her to check on something, she gets it done and gets back to me. I booked an October '09 cruise (not on Carnival) several months ago. Over the past 4 months prices have dropped several times, due to some changes I made I was entitled to yet another discount. So each time she would call and see what she could do to lower my pricing. I went from a $5,400 price down to $3,186.00 and $300 OBC. If you can find someone who will actually work with you, it means a lot.