Heavy, stiff and uncomfortable. Let's examine that a bit.
Heavy: made from heavy cordura throughout. Very durable.
Stiff: capable of carrying a terrific load without deforming into a large damp tortilla.
Uncomfortable: available with adequately padded hipbelts and shoulder harnesses of various sizes, and a stay that can be bent to fit your back. So, if you desire uncomfort, flatten the stay, adjust the harness to where it doesn't fit you, and use a too-small belt.
For up to $100 it is about the best deal going for a pack that works. Take a hiking trip and mark every strap and feature that you don't use. Then when you get home, cut those items off.
What he said. I'm not a Marine, but I have over 50 miles through the Cascades (read up up up down, up up up down) with one weighted down to 65 lbs, and it was comfortable.
