A whole lot depends upon how your GM runs spells. If they typically say, "the caster casts x spell, make your saves" then the feat is not so important, although it will allow you to immediately know when anyone is casting illusions unless they can conceal or disguise their casting from you.
As a GM, I try not to give away what spell is being cast beyond describing what the immediate and visible effects of the spell are, but when I have a PC with the feat, and I make the secret check for them, I usually also give them information like the level the spell was cast at, which also can help identify the level of the creature they are facing.