Coca-cola is a great ingredient for all sorts of dishes where you want the caramel flavoring and sugar. I've used it in BBQ sauce, pot roast, cakes, and more. Recently I was watching a food show where the chef was using it while caramelizing onions and it struck me that it would be great in French onion soup. 

The secret to a great French onion soup is to cook the onions long and low. The onions need to sweat out moisture and their sugars, thus caramelizing. You should use sweet yellow onions, but often times I only have red onions in my frig and they work just as well. The Coke just adds more caramel.

The bread is meant as a thickener. Europeans use bread to thicken many soups such as Italian ribollita or German schwarzbrotsuppe.

My cheese of preference is Fontina, but use your favorite. Some people will also add a bit of cooking sherry or wine.

Overall impression? Does the Coke add much? It adds a bit more sweetness, but doesn't really change it that much. It's more just the novelty of it.