I have heard good things about them and they are cheap; however, a good cruise-only travel agent (I found mine through the Bargain Finder feature here on CM) can usually beat them.
Regular local TA's usually don't have access to the tools that cruise-only TA's have.
Also, I've heard that with the on-line agencies, you can call them directly (and if you can't, don't book with them) and get a live associate to talk to and work with, sometimes with better prices than what they advertise on their sites.
Try a quote there and then the Bargain Finder ads here, they're totally confidential and no one gets your Email address unless you respond to them. The one I found has beat every price I've seen on the internet for the sailings I wanted, and is a total sweetie to work with as well.
I've found them very reliable. It's always a good idea to double check your booking no matter what travel agent you use. You're quoting a second hand "source" which might not be reliable in and of itself, so you really shouldn't start your reply with a big warning when you really don't know the facts for yourself!
I cruise the Emerald Princess, Eastern Caribbean on April 16, 2012