Originally Posted by
Sevarin
It cannot be used on any corpse. The Soul Inhaler almost always works on humanoid and beast corpses. It's far less likely to work on Dreadlord husks, elemental remains, and other inanimate objects left behind by other types of enemies.
You can find out what exactly can or cannot be soul-inhaled by using the item through a macro. Create a /use macro for the Soul Inhaler, place the macro in your action bar and target anything (even if it's still alive); if a white circle appears on the corner of the macro's button, then you can use the Soul Inhaler on your target (though you might wanna kill it, first).
taken from wowhead. i was pretty sure it said humanoids only at some point, but i guess im mistaken.
edit:
i personally use it in a macro with my gin-ji knife set and cremating torch to steal the soul of a burning chunk of flesh.