I went there once when I was shopping for a truck in 2009. Test drove a nice looking black 07 Ram Quad Cab, 4x4 that had like 17K miles. After the test drive, I popped the hood and the fire blanket on the hood and firewall was melted. Engine looked good, but that worried me.
Ended up going down the road to a Dodge dealership and got my brand new 09 Ram Crew Cab, 4x4, HEMI, with 17 miles, and life time warranty on the engine and drivetrain for only $3K more than what Carmax wanted for that 07. So I agree, it seems their prices are on the high end of what the vehicle is worth.