I have and wouldn't do it again. We bought a van from them a while ago.
One thing I liked about them, was that they gave us the chance to truly try out the vehicle before we finally decided. We originally drove a Dodge Caravan. We liked it on the lot, but once we put the kids in it, we realized it was too hard to get them in and out of. If I remember right, my DH didn't like the way the transmission shifted either, and that year of Caravan's had transmission problems.
We then tried out a Mazda MPV, and liked the layout much better. However, while we had it at home, we realized that it had been in a serious wreck. The back window didn't fit right, and when we started looking, you could tell it had been rolled! We ended up with another MPV that was in better shape, but had more miles on it. Because of the miles on it, we ended up in a bad position when we traded it.
I think Carmax might be right for people who are looking for a used car from a company that will be around for a while. But since we are normally new car people, it just wasn't right for us. We just weren't smart about what to look for in a used car. If you're a smart consumer, then it will probably be okay.