I think I'd rule that anyone (not just Slyphs) can use it. As a DM I like the spell, however, I would rule that you have to cast before you know if you are hit or not. This would make it possible for someone to get you to cast it with a feint and waste your spell. If they have improved feint, they could attack you that same round and get their sneak attack damage to stick (if they were a rogue).
