Not sure when I'd use it. I've already removed my cloak of obscurity from my Necro book since I never cast it; not even once. Not sure who this card is supposed to be for.
Maybe when you are getting blasted by Fireball attached to a Wizards tower?
Wizard Tower is permanent, this only lasts three rounds--including the round you cast it. And it doesn't block the Tower, it only reduces the range, and only of certain attack spells--usually the highest-casting ones, whereas the Tower is usually used in my experience to repeat-cast Level 1 (Range 0-1) spells.
I like a number of the cards in this set, and we'll see if this one sees play, but my initial reaction to this card: meh.
Edit: Oh hey look, 100 posts! Full mage! : )