Not too much of a review, but I did buy one and carry it around for a while last year. Mine is the basic Model 5 in the plastic sheath which I wore it daily on a neck chain for a few months.
I found that the tool was pretty comfortable to carry in such a fashion. However, once in a while, the hook would come loose from the sheath and I'd end up either picking it up off the ground (if my shirt was untucked) or digging it out of my shirt. Fortunately, due to the design I never cut myself when that happened. Also, after a month or so, the metal began to show some signs of rust.
As far as performance is concerned, I didn't use it much. Just for grins, I cut up an old leather belt and some seat-belt-type webbing. On those materials, it performed fine. On the leather, it would take a fair amount of pulling pressure before it began cutting, but once started it pulled through very easily. It does have a tendency to curve to one side as it cuts, but for it's intended purpose, I can't see that it would matter.
As a daily-carry item, I decided that it wasn't very useful and ended up mounting it on the center-console of my truck.
