Buying a 2008 Chevrolet Tahoe, and saving $10000 off of MSRP.
Knowing what the dealer paid for the car enabled me to accurately price out the car and give the dealership a sales proposal from my perspective. In other words, by doing my homework, the salesman could not fool nor confuse me. At all times during the sales negotiating, I controlled the negotiations. It was the most satisfying buying of a car that I have experienced.