If you're playing an Armored Vest type of deck, which basic defends regularly, this is a great way to get effectively two defenses in one (by stuffing a later attack with the Stun). One of the rare ways to inflict Stun in the middle of the villain phase, which has value at higher player counts.

Much less effective against Steady/Stalwart villains, to the point where almost any other defense card would be better. I try to build fairly general purpose decks, so I don't like to include large numbers of this in case I'm up against Thanos or whoever.