I bought my "first" vehicle with them back in CT right after I graduated college. (2010?) As someone who has never been, nor ever will be familiar with cars, it was a great, super-easy process. They took care of everything for you and my dad came in last minute to double-check. They were super patient, went over every little detail (twice) and even tagged and titled it right there for me for a couple extra bucks. I got my '08 Ford Escape for right at the KBB value and the people we worked with were awesome. However, I found out a couple years later, the previous owner wasn't a private owner like I'd been led to believe but a rental with Enterprise. I'm sure had I read the fine print I'd have seen it but I distinctly remember the guy telling me and my father it was a private owner. Clearly that term can be flexible. My bad, I'm sure on that.
Second time around, since that transmission recently died on me, I was thinking I'd go with CarMax again, especially since my dad is still in CT and wouldn't be around to see cars with me, but I decided against it. Down here in the DC/N.VA/MD area, you have SO many dealerships to deal with that I found better deals elsewhere. Also, I'd heard from a couple acquaintances that the people at the new Brandywine CarMax weren't super helpful.
I ended up using this AWESOME service with USAA (my car insurance since day 1) that helped me find my car. I think it's just called USAA Car Buying Service. Easy search tools, helps find you USAA Certified Dealers for test drives, and gives you a savings certificate...something about ensuring that you're paying the right price for the vehicle, nothing over that the dealer can try and swindle you into. Since I didn't go with CarMax this time around, it was all super helpful. It also offers some discounts on certain manufacturers too.