My impression of them is they specialize in discounts on last minute cruises. Like any other travel agency, online or offline, if you are unsure of dealing with them talk to an agent and ask them any question you can think of and go with your gut reaction.
I've never used them, but a simple search of the licences turns up that they sell cruises under a lot of different banners for different companies including Priceline and HotWire. Some others are soltourstravel, Smith Cruises and more.
In fact, it says on their website:
"World Travel Holdings (WTH)
CruisesOnly is the premier cruise brand of World Travel Holdings. Co-founded by Bradley and Jeffrey Tolkin in 2005, World Travel Holdings (WTH) is a multi-brand travel distributor with executive offices in Port Washington, New York and corporate offices in Woburn, Massachusetts. Branch offices are located in California, Florida, and Virginia. WTH has grown since its inception via three strategic brand acquisitions – Creative Leisure International, Cruise411.com, and National Leisure Group (NLG). The company's owned brands include Cruises.com, Rooms.com, Vacation Outlet, CruiseOne, Cruises Inc., and Villas of Distinction. WTH's portfolio of licensed partner brands includes BJ's Vacations, Hotwire.com Cruises, Overstock.com Travel, Priceline.com Cruises, American Airlines Cruises, Continental.com cruises, and many others."
They seem to have a very good sight but they do charge a fee of $24.99 to process your booking. I used them to find the cruise I wanted and then called Carnival direct and got the same great price without the booking fee.