We had them in a program I worked in for years up in the Adirondacks. They are a molded plastic and while they carry things well, especially bulky or hard edged items, down where the hip belt comes off the box there is a molded piece of plastic that rubs right into your lower back or hips. If my memory serves me correctly, the hip belt is attached to them and even if you take the belt off, the molded plastic remains as part of the box so there's no getting away from them. They might be more comfortable if you have narrow hips or a narrow waist but for someone my size, there's just no comfortable way to wear one.
That's all for now. Take care and until next time...be well.
snapper
PS - If you look closely at the box on the right (the green one) you can see what I'm referring to.