I am prepared to sift through as many opinions, informed and otherwise, as I can get.
Well it seems like a good thing to use. I have encountered few other slow-release sources for K. Do you have any other ideas for rock-based or organic options for potassium?
If you search here on TPT there is apparently just one member who reports using it as amendment with his soil substrate. It looks as though he has had good results, although you have to guess that the soil itself provided a robust reserve of nutrients.