I understand prices are "fixed" by the cruise line so I now book direct with them and have been very happy coupons and all. I did miss out on a cruise with cruisesonly.com so I don't book with them anymore.
I like the search engine at NCLexperts.com. But I then go to Buycruises.com and buy over the phone. It is usually a little cheaper, as they try to find discounts for you. I have even been pleasantly surprised to have them lower my final payment because they found an additional discount later on!