I have obtained excellent results with WST. It is very clean burning, and MUCH more so than 231, bullseye, and a few others I've tried. I have not tried HS6 so I cannot comment vs. that powder.
WST was developed as a shotgun powder, and isn't used as widely for handguns, though Winchester does market it as such and provides useful load data for a variety of handgun calibers with it. I have used it with great results with .38, .40, and .45acp. A friend uses it for 9mm and likes it, too. I plan on working up some .44 special loads with it this winter to feed my 629, also.
From load data I have, it is best used with standard velocity rounds and is not suited to "pushing" the maximum velocity - other slower burning powders can do that, though with larger charges and much more dirt. It is clean, very efficient (cheap), and easily available. I like it.