Two more thoughts on Hartsooks. (1) The Hartsook itself is about 2 millimeters thick. That's it. The scabbard for it is about 4 millimeters thick (except for the bulging loop they put at the tip--something one might want to saw off, if thinness of scabbard is preferable over having a projecting loop to make the scabbard lie flat when it's hanging from a neck lanyard.) This thinness is the chief distinguishing characteristic of both the Hartsook and the Hartsook/scabbard system (well, that plus the tiny-handle-plus-paracord-loop-adding-up-to-not-so-tiny-handle arrangement). The Hartsook is thus vanishingly thin and light. This makes for portability, and a real ability to keep it available in situations where you'd ordinarily not be able to keep a knife. I mean, you could use it as a bookmark. You could reinforce a hatband into a sheath and carry it there. Stuff like that.
One thing that means is that when you're holding it, a lot of your grip is probably going to be with your thumb on the top of this 2mm blade, and your index finger on the bottom--so your thumb and index finger are about one centimeter apart from each other, if your index finger is in the choil. Your hold on the knife comes from pressure between thumb and index finger, with your middle finger assisting farther back on the handle, your ring finger and pinky inside the paracord loop and applying a little stabilizing tension (and your ring finger maybe pressing a little on the back of the steel handle.) Basically, most of your grip is pressure between thumb and forefinger.
Because you're pressing on the top and bottom of a piece of steel that's about 1 cm tall and 2mm thick, there is going to be a tendency for the blade to flip sideways if your thumb and index finger aren't pressing directly toward each other, or if there's some kind of force from the side. No big deal, but with a knife this thin, it's just going to be easy for the knife to flip so that instead of the edge going straight down, suddenly the edge will be pointing to, say, the right. Keeping this from happening means applying some pressure between thumb and forefinger--more than you'd have to apply to cut with a knife with the usual much-thicker handle.
This has two effects: first, it might be somewhat tiring to cut with this thing for a very long time. Second, if you were actually trying to use this as some kind of weapon, you might find it hard to apply lots of striking force without the blade turning so that you're suddenly trying to cut with the flat of the blade. You're not going to lose the knife under this situation if you've got your ring finger and pinky through the loop, but it's not conducive to the kind of stable grip that one would want in a fight. Again, this just supports my evaluation of this knife as a great backup survival tool to make sure you've got a cutting edge wherever your car/plane/boat/pogo stick happens to break down, but if you're going to fight a mugger or mountain lion, you're probably better off with a brick or a large rock.
2. If you didn't feel like spending $20, or you wanted to go more the high-carbon-non-stainless route, you could make a very similar knife out of an Ontario/Old Hickory/Tru-Edge paring knife, by removing the wooden handle and selectively removing a little stock with a Dremel tool, to make a choil, shorten and maybe round the handle, possibly work in a Hartsook-style paracord loop, and maybe jimp the spine a bit where desired. (Like another commentator, I'd probably add more jimping on the spine than the Hartsook has, so that you'd have one continuous jimped section of spine, instead of two sections of jimping separated by a smooth portion.)