I know this answer is really late, but for those who do figure painting:
Tamiya X-21 is really great for figure painting. Add a VERY small amount to your (VALLEJO)flesh. During the painting of flesh you will have many MANY brush strokes which most of the time will buff the paint and make it shiny, the thing that we as modelers are trying to avoid.
So in short, Tamiya X-21 is great for figure modelers, but I cannot speak for it in any other capacity.