I think a lot of this sentiment comes from one dark roast in particular: whatever Starbucks brews has the nastiest, most bitter after taste. It's probably the perfect blend to compliment their sugary concoctions, since there's no mistaking the strong flavor it adds, but it's (subjectively) quite awful on its own. I find dark roasts from nearly any other brand to be more tolerable, with all sorts of interesting flavors and variations.