We've beaten this one to death.
I think most of us don't like the idea of 15% being imposed upon us as restaurant patrons. Most of us are generous (or at least fair) tippers when the food and service are very good. The problem arises when it is not. Most of us will not cause an awkward scene with restaurant management but will simply not return nor recommend the restaurant to others. I think this is still the most effective course of action.