I have only purchased ONE vehicle in the last 20 years that did NOT get financed via Navy Federal, and going with that finance corp (Ford) was the biggest mistake I ever made. I've heard similar horror stories from others about GMAC, Toyota, etc...
Bottom line--I walk into a place knowing what I'm going to pay and what my trade-in (if any) will/should bring. If they can come in under that, I give the dealer the draft given to me by NFCU. If they can't, I find a dealer that can.