My niece recently used them and was quite happy. They gave her high blue book on her Toyota Corolla. They did mention to her that is not always the case. They adjust their offered price based on the popularity of the particular car, as well as condition. She said they took about 45min to drive it around and inspect it, then gave her the offer, which she could redeem at any Carmax location within 7 days. She took their offer right then, no muss, no fuss.
I'm going to take my Trooper to them when I'm ready to sell it, just to see what they'll offer. If it's close to what I could get myself, I'll probably let them have it to save myself some hassle.