Not that comfortable
I tried this on a 4 hour flight. My feet ended up stacked on top of each other, not really possible to keep your feet side by side. If you try to put it under your calves your legs are  too high to get your feet under the seat in front of you, especially if you have a carryon under the seat in front of you. I had a more comfortable effect just pulling my carryon out a bit and putting my feet up on that.
Rating: 2 stars