I understand that it's a violation, but we haven't seen the case where an armed civilian in violation of any posted policy has effectively stopped a criminal.
Probably depends on the jurisdiction, but the first shooting by a CCW in Wisconsin took place about a year ago, when an armed robber with a shotgun was engaged by a citizen standing in line, resulting in the criminal suffering a wound and being neutralized. This took place in a larger supermarket that had "no firearms allowed" signs posted. The Prosecutor declined to present any charges.
Actual mileage will most obviously vary, and no general observation is offered.