I have now sold two cars to Carmax and both times the experience was favorable. It takes longer than I expected to sell the car (and buying a car always takes me longer than I expect too, I wonder why my expectaions are so far off). They also always seem to ask for documentation that I don't have with me and I have to go home and get it. I think the documentation they need varies from state to state, so I won't list it here.
The first time I got an estimate, but didn't sell my car immediately. It took about an hour for me to get through the line and get the estimate. I then had to get a replacement title (the one I had still listed a leinholder) Actually selling it took another hour and a half or two hours. They had to do a re-inspection to make sure I hadn't damaged it in the intervening day, then officially buy it, then I had to wait around some more while the made out a bank draft (like a check, but subtely different) In the end, they offered a little more than I expected. When I bought my new car, they had asked if I wanted to trade in, and when I said I was going to get an estimate at carmax, they told me that carmax usually could beat their offers.
The second time, there was again missing paper work, but I didn't bother with getting an estimate first, since that didn't seem to save me much time. When everything was in order it again took a couple of hours to get through everything, and they again offered more than I was expecting.
Aside from the time it takes, and never seeming to have the right documentation, selling a car to carmax is fairly easy, and I would recommend doing that instead of trading it in when you buy a new car. I haven't bought a car from them, so I can't tell you if the prices are fair or how long it takes. Also note, that if you sell them your car, you need an alternate way to get home.