Depends. I actually had a really good experience with Carmax. I had a 2002 Jeep Grand Cherokee back in 2005. It was a piece of crap, lots of things in need of repair with only 30,000 miles on it. I bought it used from a Chevy dealer with only 12,000 miles on it and it was three years old. So out of warranty. Long story short, I wanted to replace the piece of crap jeep with a Nissan Xterra and Nissan wasn't willing to give me much for my trade in on the Jeep. So they suggested I go to Carmax, sell the Jeep, and come back and buy the Xterra.
While waiting for the offer for the jeep, that was about $1,000 better than Nissan, the sales guy at Carmax found almost the exact Xterra on their lot for about $5,000 less than what the new one would have cost me. It was a year old with 10,000 miles.
I've had nothing but great luck with the Xterra and coming up on 120,000 miles. So I saved at least $6,000 by going to Carmax.