Costco/truecar/usaa all use the same (truecar) back end. Though I believe the price can vary slightly, or at least did when I used it about 3 years ago before it became more popular. When I bought my jeep a few years ago I used the USAA service as a starting point, and did a tiny bit of negotiation at the dealer on some add-ons, etc. Based on a lot of research, I'm pretty sure I got just about the best deal that could be found at the time.

I think it's a great way to get started if nothing else. Up to you whether it's worth fighting for that last little bit of money at the dealer, and if your time is worth the hassle of playing dealers against each other, etc.