It depends.....a good agent can be invaluable if they know the ships and ports and can go the extra mile.
I've found, lately, as a repeat customer of certain cruiselines, that when I login I get much lower fares that offered straight to the public.
Clearly, before you commit to buy from an agent, compare the cruise line website direct and other internet cruise outlets.
For the most part, we have not seen cheaper rates than the cruise lines offer direct when we are logged into our cruiseline account.