This is a steal at this price. I'm still really enjoying this game having gotten it at release. Its not without its flaws. If you don't play it stealthily its probably terrible. But as a pure stealth game, I love it. For $30, a next gen game thats relatively new, thats great.
If this keeps happening though, it'll be tough to argue why people should preorder a game at full price when it goes on sale for 1/2 price less than a month from release. Preordering a game already isn't really worth it (almost no benefit because the preorder exclusives are usually garbage and there is always risk that the game will not be what you expect). Now add quick price drops like this, and preorders could be in trouble in the future.