As with most things, it depends on the dose and length of exposure. An occasional soda is harmless. If soda is your main (or only) intake of liquids it's very unhealthy. It's loaded with sugar which leads to weight problems and possibly diabetes. Drink mostly healthy stuff like water, milk, fruit juice (again, not the stuff with added sugar) and ONCE IN A WHILE a soda.
Too much of it, yes. It doesn't matter whether you drink regular or diet, soda should be considered a treat. The sugar in regular soda promotes weight gain and associated illnesses. The artificial sweeteners in diet sodas pose their own set of risks. And both contain phosphoric acid, which is bad for teeth and bones.