Are junk-food taxes effective? Yes.
Experience from Hungary and Mexico show that these taxes do drive down consumption of sugar-rich food, especially among people who eat a lot of it.
But is it enough to combat the obesity epidemic? No. Healthy foods also need to become more available, and a possible solution is subsidizing differently. Stopping subsidies of corn, wheat and soy (crops used intensively in junk-food production) would likely have a big positive impact.