Yeah... I know Casteline - haven't tried it myself, but have heard good things about the older stuff and not so good things about the newer batch. Always wanted to try it though. Is it true that it is a bit tougher to work with, not as soft, as super sculpey, if you have tried that and can compare the two products?
I see you are going for the ESB look for the Vader... and it may just be the angle... but I'm pretty sure that the chin vent is more RotJ style than ESB... though, it is a minor thing... and your sculpt definitely looks good.
Keep up the good work.