It works as advertised. 10-15 fps increase in velocity, with little difference in pattern. The problem with the stuff is that it is a mess. DO NOT RUB YOUR EYES NOSE OR FACE until you have washed your hands. This stuff is mica, a mineral that is slick on the flat surface but microscopically jagged on the edges. It is softer than other components so it gives lubrication to the wad coming out and going down the barrel. It has been around along time, so some must use it.
Let's assume it works. Now, what benefit could a 1% velocity increase provide? That's less than the typical tolerance on the claimed velocity on a box of new shells, and reloads are typically less consistent than facory. So, if the third and fifteenth shell in your box aren't withing 15FPS of each other, what good would it do to have both of them shoot 15FPS faster?
If you want an extra 15FPS, run a hone thru your powder bushing for five seconds and be done with it.