G'day James,
people I know that have used them swear by the gear for it's ability to assist horses that go a bit too straight behind.
After having a look at the gear and understanding the methodology of how it's supposed to work, it seems to me just like the spreaders that were invented ( actually spreaders were an innovation rather than an invention as they copied the "pneumatic tube" that people were using)about 35 or so years ago, that it will assist some horses but obviously not all.
I have a square gaiter that may be assisted by one of these and believe it or not went and got one today ( thanks Garrards)....I'll let you know in a week or so how it is suiting this particular horse.
Cheers,
Dan