I love Costco and shop there frequently. But, not for my cars or trucks. They are arguably no better or worse than a dozen other "buying services" that are parasitic to the new car sales industry. They guarantee a price to the consumer which is never the lowest price in the market nor is it usually any lower than other services. For this they charge a monthly fee, simply to not be in violation of most state laws, and it averages $400-$500 per car delivered in the program by the participating dealer. This is a national average which dealer associations track.
For the consumer who prefers not to do the research or the shopping necessary, it is a viable and satisfactory alternative. You simply are not going to get the best price and should not be under that illusion.