The only time carmax is even worth a look is for an extremely rare car, because they will ship cars relatively cheap cross-country for you. The problem is, the more rare the car is, the more shipping is. Also, their prices tend to be $1k - $2k above the price you could get from a used car lot.
I did glance at them when I was looking for a car 5 or 6 years ago. The more common cars were available to be shipped for $0 - $250. The STIs and others of similar demand and rarity were about $800. That, and Carmax's prices always approached the prices the dealers were looking for.
I bought a motorcycle instead.