I know quite a few people at work that have used CarMax. I also know a friend who works at CarMax. For the most part people are happy with them. It seems sometimes if you look around you can find a better price. There biggest thing is getting a car that has not been in a major wreck or a lemon. They also buy cars from people, my friend says they give quotes 50 to a 100 times on used cars on a Saturday. He also says they make about $1,000 on average on each car. They move them in and move them out.
A co-worker had trouble with his Saturn and CarMax would not fix it even though he had a extended warranty. He went to another repair shop and then went to CarMax and got a new head gasket, a common problem with that type of car. So the warranty is so-so.
I am a little luke warm with them, but if you want a good car at a decent price that is in good shape and your not that knowledgeable on cars, it maybe the way to go. Another co-worker just bought a used BMW 3 seires car and he felt the price was good and the car was in good shape. ( I just added up in my head and I know 8 people who have bought from them.) I also went into the store to sell a car of a friend who passed away. I took his Wife and they had bought the car there years before. Since it was bought from them, they knew it had not been in a wreck and they gave us more money than I thought they would. I felt they treated us right.