I've had a couple of different experiences with Carmax. My first was the purchase of my VW GTI, I knew exactly what I wanted, test drove and bought it in less that 45 minutes....INCREDIBLE! I loved that there was zero waiting, zero pressure, zero negotiation and FABULOUS interest rates, about the same you'd find buying a new car.
My second experience was selling my Corolla, it had 200k miles on it and the dealership had offered me $700, took it to carmax had it appraised, they offered me $2,700.
My third experience, I had no clue what kind of car I wanted, went to carmax wandered around, was completely overwhelmed and felt more informed about the different models than the dealer.
So overall I would absolutely buy a used car from them again but I would narrow down my search before I went, its definitely hassle free, quick, easy and it makes buying a car!
In retrospect I would have gotten their extended warranty, especially on cars notorious for expensive repairs, it was only about $1500 for another 50k of coverage and it would have covered thousands of dollars in repairs for me.
Got there with no particular standards in mind. Plenty of sales staff to greet you at the door. But I more felt like they wanted me in and out as soon as possible. Staff did not seem to know much about the particular cars... although there is some room to be forgiven considering the vast variety. Still, it does not help me buy a car!