In one case, Automatic Matching increased the cost of a particular ad group by 600%. About 88% of all resulting clicks came from the automatically matched keywords--and yet only about 30% of the traffic the ad group generated was actually relevant. All those wasted, poorly targeted clicks "effectively made the CPC of the keyword buys in this AdGroup 3.5 times more expensive with Automatic Match turned on," Gilbert and Churchill said.
So keep an eye out for the Automatic Match switch to be turned on in the coming weeks, as the feature will be enabled by default, as per an official Google email. "Automatic Match does not start spending immediately... once activated, it takes up to a couple weeks for it to learn what it thinks it should do," Churchill and Gilbert said. "So... Keep tabs. It could take off like a rocket at any time."