I just used the USAA car buying service in December to buy a used car. I had both good and bad experiences.
On the good side, it was a good way to find a used car in the price range I wanted. USAA's negotiated deal was $100 off the dealers price.
On the bad side, I made the assumption that the dealership and the cars had been vetted. Wrong assumption. I paid to have an independent mechanic inspect the car. He identified several issues about the car, two of which I considered important enough to present to the dealer. They said they wouldn't fix the problems, but would give me an additional $250 off the price. Checking with a VW dealer parts/service station, I found out it would be a little over $1000 to get them fixed.
Then when we were signing papers they offered me a 1/4 % better financing than USAA, which I accepted. But I understood that a 'gap' insurance policy was included. When I reviewed the papers later, I discover that it accually increased the purchase price by $800. So I called and cancelled the dealers financing, and went back to USAA's financing, since they only made a onetime charge of $297 for the same 'gap' insurance. But, when I got notice of what USAA paid the dealer, it include the negotiated price, plus the supposedly cancelled 'gap' insurance.
So shame on USAA for sending the dealer a payment I had not approved or agree to! Then, 5 days later, the car wouldn't start because the battery was dead!
I finally got things squared away with the dealer, including a new battery that cleared up one of the original sensor problems. And I got the $800 charge applied to the loan, but USAA wouldn't renegotiate, so I'm stuck with the original inflated loan payment.
My conclussion is that though USAA touts the USAA car buying service as a 'benefit' to it's members, it's actually a benefit to them and they pay for the service from some 3rd party vendor and we end up paying for it in reduced profitability or increased premiums.