Yes, definetely depends. We took my wifes Grand Caravan there back in June when we were initially thinking of dumping that for a Suburban. It's got low mileage, clean history and impeccably clean. They offered us $9K; KBB was a little over $11K. Two weeks prior to that, a GMC dealer offered us $85 for the same van on trade. The salesman said they sometimes tell customers to take their cars to Carmax instead of trading them at their dealership.
We ultimately decided to keep her van and ended up trading my Safari for a '07 Yukon XL with 14K on it. I had my Safari for sale for over a month with out one call. Granted it had 130K, but it was clean and ran like a top. We ended up trading it for the Yukon for less than what we initially wanted for it, but we bought the Yukon for a lot less than we planned to spend. So in the end, it was a wash. The deal was within the budget we had set, so we decided to go with it.
My experience with Carmax is that they're high priced. We found similar Yukons and Suburbans elsewhere for a lot less, with lower mileage.