Any feedback is valuable! As a reader, I know that most people don't come back to leave feedback because of all the reasons mentioned above--podfic is portable, so by its nature it's listened to away from an easy means of leaving feedback. So when someone does come back to leave a comment after listening, I treasure it, even if it's just "Thanks, I loved this!" I know it takes extra effort to leave feedback for a podfic, so I'm pleased whenever someone takes that effort. :)