I also just ordered a truck under the Costco program. I had sat down at my computer and tried about 6 Ford dealers near me, going straight to the 'internet sales manager', also tried using an internet car broker (Carsdirect), and my credit union's program. Then I tried using Costco's program. Costco was the cheapest. The dealership's salesman in charge of that program said 50% of his business came thru the Costco program and that he had to 'aggressively' price under that program. Rebates and any special financing were in addition to the bottom line price.
The one drawback is that the dealer you must use may not be local.