I have the Marbles knife. It's pretty good overall. At least as good as an original, though that isn't really saying much. Hefty and well assembled, the knife blade seems to decent steel, along the lines of Rough Rider blades. Fit is nice, all the backsprings have perfect amount of snap. The nail nicks are all knurled, much like Rough Rider knives often have. And unlike any original demo knife I've seen. The whole knife is fairly well polished, with no grind lines visible on any of the blades or even on the outer handles, despite the stamped indentation pattern.
Some minor criticisms: The screwdriver tip on the bottle opener blade tapers to a fairly thin, and slickly polished end, much like you'd see on a Swiss Army Knife. It would have been nice if it were squared off like a normal flat driver, and maybe left a bit thicker for extra rigidity. I was also curious about the backsprings, as the originals often seemed to have fatally tempered backsprings, prone to snapping. So I opened moth blades on each spring half way, which forces the springs to flex to their maximum. BTW, this is something you should probably not do with a knife you care about, to avoid possible damage. Anyhow, the result was not a snapped spring, but rather, a bit of laziness, indicating the springs deformed a bit, losing their original set. This shows that the backsprings are probably not as brittle as the original spec GI/demo knives.
My conclusion is that this is a fairly worthy knife/tool, probably better overall than the original that inspired it. And they certainly can be found inexpensively.
FWIW, the backsprings were restored to more or less original springiness using the old strike-the-backspring-in-the-inside-at-the-right-spot trick. I've actually grown to like this knife and it sees some pocket time. If I were to get another, I wouldn't pull that spring durability test again, as it's wiser to not tempt fate. If I have some time tomorrow, maybe I'll dip it on the ocean and leave it wet to see what comes up in the way of rust and corrosion.
Does anyone have experience with the Marbles GI Utility Knife? I know it is not the same as the military Demo Knife, but it seems to be the same pattern. I am looking for a rust-resistant utility knife good as gifts for villagers I worked with in Samoa.