Like the name suggests, its key feature is four strips of bacon coated in a seasoning that includes brown sugar and black pepper. Other than that, it comes with egg and a slice of American cheese on a croissant.
The black pepper bacon is also available as part of a Wake-up Wrap, which has a smaller amount of the same ingredients folded in a flour tortilla rather than on a croissant.
I had the opportunity to preview the sandwich earlier and thought it was pretty well-balanced and just sweet and peppery enough without being overly so. I also got a look at how they prepare the bacon:
Nutritional Info - Dunkin' Donuts Sweet Pepper Bacon Breakfast Sandwich
Calories - 560 (from Fat - 300)
Fat - 34g (Saturated Fat - 14g)
Sodium - 1030mg
Carbs - 43g (Sugar - 8g)
Protein - 22g