We had a GREAT experience at Carmax. We were able to research online, bring a list of cars we were interested in to the dealership, hand them to the dealer, and he took straight to those cars. DH and I both haaaaate the whole haggling process, so for us, it was a painless process.
As far as the car itself, we've had great luck. We bought a 2000 Pontiac Grand Am GT1--it had ~30000 miles on it at the time. I think we bought it back in 2002, and didn't spend a penny on repairs till this fall, when we had some misc. repairs done. We also opted to pay for the extended warranty (through Carmax) and that ended up covering the entire bill, save the $25 deductible.
We went through a dealer to buy my current (used) car the following year, and I have kicked myself ever since for not going back to Carmax. The process was stressful, the people were unfriendly, and in hindsight, we really got hosed on the purchase price.
Bottom line---there are risks with every used car, but I have every intention that our next car purchase (we'll only buy used) will be back at Carmax.
Geez, I sound like a salesperson--I'm not, I swear!! They were just really great!