I quite often have that spell ready if I'm in human form, do love it. It seems that some people are utterly fooled and others spot me straight away. I can't quite work out whether that is because the item and area aren't a good match sometimes, or it's more about whether the invader is paying attention.
I got spotted (as a tree) regularly in the Forest while waiting for invaders. Not sure if your character still casts the light they always have whilst disguised, but I obviously stuck out a mile. In Anor Londo, as a pot, I quite rarely get picked out however.
What other game developer would have thought to put something so ridiculous (and brilliant) in?