Are Shakti Stones effective?

I have never used Shakti Stones and wonder if they really work and, if so, what do they do,  I have used the Shakti Holographs and found them to magically focus the sound and tighten the soundstage. From what I have heard, even from Shakti, is that the stones make a significant difference is some usage and less in other. Has anyone used these and what do they do?