I've had two personal experiences with Carmax and their buy offers.

The first was after my brother passed away. I needed to sell his car which was in ridiculously bad condition. The offer they made was more than fair in light of its condition and I took the money and ran.

The second experience came when I was shopping for my Crossfire. I had an Acura Integra GSR that was in immaculate condition. The local Chrysler dealer gave a trade-in allowance that seemed way too low but they wouldn't budge. On my way to a Chrysler dealer about 15 miles away, I stopped by the nearby Carmax just to see what they would offer. After about 15 minutes, they came back with an offer that was a few hundred dollars ABOVE the KBB value for my car in excellent condition.

Armed with that quote, I was able to negotiate a MUCH better deal at the next Chrysler place. And the rest is history.

Overall, I am very impressed with Carmax's car buying system.