I've never used CarMax but I'd definitely consider them if I wanted a used vehicle. I'm in a similar situation when it comes to keeping cars a long time. If anything, I tend to keep them for an expensive repair job too long.
It does make the car buying experience easier. I know the trade-in isn't worth much and I can always just donate as a fall-back. I also haven't needed to finance for a while. When you can just basically ignore the dealer number BS and go in with a take it or leave it price based on the invoice, the experience may still be annoying but it's not particularly stressful.