As Mike said, their prices are exactly the same as any agent will offer you, so it's better to use an agent you can talk to, who knows you, has been on cruises, and provide personal expertise. Most of their people work in a call center and are not certified travel agents - they are just there to sell you something, not to provide you information.
And as was stated, ignore their "up to 75% off last minute cruises." They are referring to 75% off the brochure rate and to me this is false advertising because no one, and I repeat, NO ONE sells cruises at the brochure rate. Not even the mass-market cruise lines sell their own cruises for the brochure rate. Their prices are exactly the same as other agents, so they're not offering anything others can't offer you. But others can offer you better service.
So my best advice is find a reputable agent you feel comfortable with and avail yourself of their services. They'll work hard for you. Plus, should you have any problems, they have contact information not available for you and that's when their assistance can be invaluable.