Carmax gave us full blue book trade-in value for good condition on an '01 PT cruiser, that had an entirely rusted out (and clogged) cooling system (which we disclosed, repeatedly). I would have rated the car as "Fair/Poor" myself, just because the repair estimates for a new radiator, all the water hoses/pipes, a new pump, and getting the block taken out, and cleaned out was going to cost almost the exact value of the damn car. And they sold us an '05 Honda Oddesy at Blue-Book retail that they delivered to the location from another state just for us to test out.
I was on Edmunds on my phone's web-browser the entire time to double check.
All in all, I'm happy with the experience. And as others have said, no hard-sell on the "extras". Nor did they do that insulting B.S. of trying to get a monthly payment figure out of you that you can afford first, then working backward from that to try and set the price of the car.
I did read all the complaints about carmax on the internet, but then again, there really isn't a "national brand" of used car chain to collect all those complaints either. Somewhat akin to the Glock KB phenomena IMO.