Coffee = Fantastic
Chocolate = Divine
Coffee + chocolate = not so much.
I'm sitting there drinking it, a little bit confused: do I taste chocolate or do I taste coffee? The two flavors are competing, and not in a good way. In fact, the "new" flavor is so overpowering that the original thing I liked about each individual component (uh, that would be the sugar and the caffeine) is lost.
I like milk and I like orange juice, but I don't mix those together either.
Some things are better solo.