Thanks to Jewel-Osco's Saturday Sampler, I tried Coffee Coke. At first blush, the combo was pretty good. There wasn't much coffee and what was there had sort of a flat, dark, espresso-y flavor. Inoffensive, not much different from regular Coke. Foamy like regular, too. It did have an odd herbal aftertaste, which I assumed was coming from the coffee.
Then I read the nutrition facts and ingredient label. First up, 70 cals for a can. That's about half normal, so I figured they had mixed half no-cal coffee with regular Coke and carbonated to the normal level. Unfortunately, no. It's partly sweetened with sucralose and acesulfame. To me that's the worst of both worlds. It lacks the advantage of being sugar free and it has the aftertaste and wrong mouthfeel of artificial sweetener. It's also obvious why the coffee taste is flat: They use coffee powder, which I assume is another term for instant coffee.
Final thought: I would buy this if it were fully sweetened with sugar (preferably not corn syrup), but only if it were the same price as regular Coke. Or I might mix my own with brewed coffee and recarbonate.