When we bought our 2015.5 GMC sierra denali I went to our credit union buying service and Costco. Ended up with the credit union. slightly better price than Costco, but also much faster response in finding exactly what we wanted and getting it. I gave costco dealer sales guy and credit union what I wanted, which was not in any local lots but a few hundred miles away.
Interestingly the credit union and costco went through the SAME dealership. Costco through a regular sales guy, credit union through fleet sales.
I know others who used costco and were very satisifed. Had the costco dealer regular sales guy responded quicker I might have gone with Costco.
I don't like haggling, IMHO if you really want the very lowest price, willing to spend time haggling, going around dealing with sales guys you probably can beat Costco, etc.
If you want a very competitive price, no hassle, nice treatment, can't beat Costco or credit union car buying services.