On the face of it, that would seem only a costly annoyance. If, however, it were only a practice run for a larger scale attack, then the implications are chilling. It's one of those "what if" scenarios played out in real life.
This, in conjunction with disabling the electrical grid, would cast almost any large modern city back to the stone age. No water, no communications, no fuel for vehicles, no air travel, etc. etc. Were those things to happen to a number of cities at the same time, it would be devastating. No foreign power would need to invade, really, because they've already shut us down.
Suddenly, you really are in a survival situation, a much more likely scenario than nuclear explosions or space invaders. Almost any terrorist nation could (cheaply) mount such an attack. Scary, huh?