I've used COSTCO for cruises and all inclusive resorts in Mexico. Price and service is fine. I think their cruise prices is probably better then the majority of TAs (possibly excluding a TA who has a group booking). Shop enough and you'll find someone who'll offer a slightly better deal.
Cruisecritic doesn't want posters discussing specific TAs. There are relatively few posts about COSTCO, pro or con. I respect cc policies and would never start a thread about COSTCO travel.
There shall be no third-party recommendation of travel agencies on any forums within Cruise Critic. General discussions regarding the TYPE of AGENCY to be used to purchase a cruise will be allowed, as long as no recommendations for any source of purchase are posted.
People who've had favorable experience with a TA, and who respect CC rules, won't be posting.