I used CarMax for their car-buying program.
I went to CarMax really wanting to hate them because they're the "big box" version of used cars, but everything about the experience was pleasant and I would happily go there if I needed the service again.
I went on a weekday evening without an appointment. The staff explained the process to me and asked for my keys. They did a visual inspection and then drove my car around the block.
Following their inspection, they input all of my car data into the computer (took about 20 minutes total). The guy working with us explained what data they collected and showed us the offer (which was very competitive after doing research via Kelly Blue Book). He gave me a printout of the offer, which was good for 7 days.
While I was waiting, the lobby was clean and comfortable with complimentary drinks. I was also able to browse cars for sale on the lot.
Overall, it was a great experience!