Not yet, but if i do decide to go newer used car then they are one of the first places i would like to look because:
1) good quality of cars---generally no issues
2) decent warranty
3) you can return if within 7 days if you don't like it for a full refund (will have to double check this as the current policy may have changed)
So, the prices may be higher than if you found an equivalent car from a private seller, but carmax takes away all the hassle factor---this may make the higher price worth it.