I've sold two cars to CarMax and was well-treated both times.
It's been posted elsewhere, but my most recent CarMax experience was with my first Model S. They gave me $80k for a blue 85 with a pano roof, leather, air suspension, and obeche matte trim. It was in very good condition, but was 7 months old and had 9k miles on it. They paid more for it than I paid (after the tax rebate)! It was later acquired by someone in Las Vegas who paid about $800 more than I sold it to CarMax. I don't think CarMax will do it again. When I dropped it off, it was clear that they recognized that they would not be able to service it so their used car warranty would be a problem.