I've tired the USAA services numerous times. Each time, I was able to get a better deal walking in cold and negotiating.
In addition: The local dealers get your contact info - so they call, and call, and call, even after I bought a truck from another dealer. I once had to cell the general manager to get a salesman to quit calling to sell me a GMC 1500, when I wanted and bought a GMC 2500.
Same thing with insurance - USAA might be the best customer service (Lots of folks says that) but none of what they do it the least costs. So, make your choice knowing that. :)