Faust's dust is mostly safe against moves with short active times and long recovery. Johnny has many of such moves.
Of course Faust's dust should be used rarely. It's really slow and somewhat telegraphed (has obvious-looking animation).
When there's hammer/tarai/mini something/meteor. Use it like you would use his dust (rarely).
It's "practical", because his j.D is somewhat safe (it's hard to beat). But I think you will rarely break anyone's guard with it, since the move isn't that fast.
I never heard about it. The items should be totally random.