To be absolutely honest, my wife and I only went to Carmax to test drive a bunch of used cars with the intention of buying the one we liked somewhere else for cheaper. We did find cheaper cars but returned to Carmax for our purchase and would do so again without hesitation. The biggest reason is this: While their used pricing is on the upper end of the used market value spectrum, their cars are as close to perfect as they get. And you have 30 days to make a list of everything that's not perfect and have it fixed. Here is a small example but representative of the whole experience: after deciding at Carmax we wanted the Nissan Pathfinder, we went to 4 Nissan dealerships to look at 'Certified Pre-owned' Pathfinders and found that, while a little cheaper, most were missing floor mats, manuals, DVD remotes and cup holders -- all items which those dealerships offered to sell us but not include. Not a single car we test drove at Carmax was missing any of those things. In fact, when purchasing a car, they give you a notice advising you that they guarantee a car manual and will purchase or reimburse one if your car does not include one. 3 days after our 30-day Carmax warranty expired, the car began making a squeaking sound. I called, ready for a fight. The service advisor's exact words were, "Do you really think we would turn you away 3 days after your warranty expired?" He laughed, told us to bring it in and they fixed it for free, along with 2 other preemptive things they noticed (rear hatch shocks and a tire pressure sensor). All under warranty.
I do highly recommend you get financing somewhere else because their percentage rates are insanely high (our sales person even warned us of that, which is another thing I love).
The Carmax way is "this is the price, take it or leave it," which takes some getting used to, but you are in great hands.