Have used it be careful of add-ons (hidden charges) especially if you are bidding be sure you know all extra expenses before hand - look carefully. Having said that - bid and won a cruise (10 day)couple of years ago,total cost for two not including air to San Juan, was $576. pretty good I think.
Also did Banana Beach 2005, think price ended up close to the same as booking direct mighta saved couple of hundred bucks.
Probably wouldn't do booking again but would certainly bid on cruises or packages.
Capt Jeff might know more and his business may save you just as much. IMHO