Special Dark is a Dutch process cocoa, which means that it has been processed with an alkali. This affects the cocoa's interaction with leavening agents; Dutch process cocoa will not react to baking soda and so it can only be used with recipes that call for baking powder. In general, its taste usually is lighter and sweeter and has less bitter or sour notes. Hershey's Special Dark, however, is intensely, quietly dark, like an Oreo cookie.
That black, rich, and sweet cocoa taste is not suitable for everything; the lighter or fruitier notes in regular reddish-brown cocoa will be missed if you use this in some recipes. But I made chocolate cookies with this last week and they had the chocolatey sweetness and black crumb of Oreos. This won't replace my regular cocoa powder, or even my Droste Dutch process cocoa, but it has earned its own place in my cupboard.
• Hershey's Special Dark Dutch Process cocoa available in the baking aisle at most grocery stores