I've never used one.. but after a recent 15 hour flight from LAX to SYD.. i'm beginning to think about it.
I'm not skinny or obese.. but i did sit in the bulkhead row which is a bit harder than usual perhaps.
I did walk every 4-5 hours.. just to stretch.
without going into TMI, let's just say i had skin chaffing and intense bone pains...
enough for me to ponder about a foam as well