I used costco auto, and it was something like below $1500 invoice if I had used finance service other than Mazda's 0.9%. The email reply said "in lieu of mazda finance". I didn't know what that meant until I was talking with the dealer.
Before all this, I tried negotiating through internet quote but none (out of three) of the dealers were willing to go to an acceptable price. I got tired and tried costco auto and was surprised how easy it was and with good pricing too. No haggle at all.
2014 GT AWD, no tech, $30215 msrp, $29114 invoice, $28288 paid ($826 Below invoice, no tax in OR) in end of December 2013. By the way this was the price one of the internet sales rep (before I used costco auto) also quoted me after several haggling over the email. So I think it's worth using costco if you don't want to waste too much time, and pay reasonably but it won't be the best deal. As a bonus, I also got a $50 coupon (can be used to buy parts) for doing a survey for costco auto.
Next time I purchase a car, if I have time, I would first try costco pricing, use that as a "good price", then try internet quotes to several dealers.
edit:
To use costco auto, you have to go to costcoauto.com and create a ID and start from there. The dealer will contact you through email.