I love Costco.
I bought a car from them in 2008. It seemed like it was very easy to get a very fair deal. If I had wanted to put more work into it, I probably could have done a little better but was too busy to bother at the time.
From the folks I've talked to more recently that have used their various programs, that seems to be about par for the course.
They do have some super specials go by here and there that involve kick-backs with Costco cash cards and stuff, lined up every now and then with a manufacturer's promotion that might be hard to beat if you value all that is bundled in there. Short of that, I suspect you'll get 90%+ of the deal with 5% of the effort. If the last <10% of the deal is your thing, knowledge is power and there is so much of it running around out there you can probably do better or do similar with another buying service, but you'll have to work at it.
This is my signature block. There are many like it, but this one is mine.
My signature block is my best friend. It is my life. I must master it as I must master my life....