If you ask me it is indeed quite useless. If you want to steady your equipment while not using a monopod, tripod or any other form of mount, it is important that you find a pose which enables you to relax your muscles. In other words, and I am speaking as a marksman for which the same rules apply, using your muscles to apply force (you have to keep the wire under tention) will only make you even more "unstable". And the longer it takes before you take the picture the more unstable you will be. Much better of with a monopod or tripod. It might not be as small as that "thing", but much more reliable! Carbon is a nice upgrade if you can afford it, else just take aluminum. Personally I think alu is just slighty more stable, but the downside is that it's more heavy ofcoarse!
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I just can't totally agree with you on the sting, although you may have noticed I'm going the home made, cheap option to try it out again before I consider something commercial. The reason I disagree, partially, is that I find heavier gear to be more stable...i.e. a light weight camera body with a small lens will shake in my hands more than a heavier body with battery grip and heavier weight lens. There is limit though, and beyond that you will induce shake. However at long as you can put some tension against the string, your ability to move is limited to an arc, like a chunk of a sphere, which is exactly the same as a monopod. Except with a monopod, if verical, you only need to apply enough force to keep its position, which with the string you need to apply slightly more force than is necessary to just hold the camera.
As several people pointed out, a good three point brace, such as a camera strap running around both your elbows (3rd point is your hands/camera), works very well, but obviously is limited to your ability to not sway. Its also based on the idea of continous light pressure to maintain the tension in the system, but not so much you exhaust yourself.
