How many uses can you get from the Dust of Disappearance? The cost of the item is 3500 gp. Also, this same question was asked before, but I was not sure of the answer. If it is only once, then the cost is very prohibitive.
The big thing about it is enemies will have a hard time trying to remove it's effects once they're in motion. Most of the standard invisibility debuffs can't reveal a person under the effects of the dust.