I enjoy reading the web forum for servers in the U.S. and they've been talking about this. One guy reported people showing up in pajamas in his area Denny's. Another person who works for a different restaurant NEAR a Denny's reported overhearing her customers: they got the overflow crowd and some people were chatting about how they were going "Denny's Hopping".

Denny's is known for being cheap eats, and there's nothing wrong with that aside from the fact that they often to draw the less classy members of society. A campaign like this only makes that worse. When you advertise something is FREE, be prepared for the vultures to land en masse.