I passed though a checkpoint recently and after screening was done, the last thing each pax passed as they exited the TSA area was a large stand with a poster inviting feedback. One either side of the stand was a holder with plenty of preprinted comment forms and there was a drop box in front if you wanted to fill out your card right then and there if you wanted to drop in your card, or the address to mail in the card was printed on the form.

I don't ever remember seeing such a good set up for true feedback. ^

The airport was PHX - Phoenix, AZ (NWA gates)

I can also add that I have been refused comment cards at BDL, MKE and BWI in the past year after asking for them.