Me... I do repair work, and re installation, the odd re stretch but no new installs.
I'm skeptical that it will work on products that have medium to heavy weight wool face yarn with a jute backing. I get the 'stretching through between the legs' concept, but the demonstration is all on what appears to be 'builders grade' carpet, lighter weight cheaper products that easily skew out of shape to stretch through the legs.
I can see definite benefits to having it as part of the tool box, for places where you can't stretch off an opposing wall/fixture, for re installation after flood work, for seam repairs where you want to stretch up to re seam, larger repairs etc.
As an inspector also, I strongly suggest and recommend the use of power stretchers to attain sufficient stretch on an installation, many of my inspections are lacking as installers take short cuts due to time, or difficulty with angles, unsuitable stretching platforms etc
John