I eat at McDonald's about twice a week here. Part of me feels like I am the typical American tourist treating McDonald's like my personal American embassy, and part of that may be true. But let me tell you why McDonald's > everything else.
1. It's cheap. I live in an expensive, touristy area and it's often the only thing I can afford.
2. It's familiar. A burger is a burger is a burger, it tastes the same and it even has the same name. No matter how discouraged I am about my French not being up to par, they always understand me at McDonald's.
3. McDonald's here (at least on the Champs Elysees) is ballin. Seriously. There is a separate counter for coffee and espresso and pastries (macaroons, croissants, apple tarts, flan, muffins, etc. Think the Starbucks snacks but cheaper). They have 3 chocolate mousse for dessert. They even have BEER here. AND, on top of it all, there are no open container laws in Paris. I can get my 2 euro beer and then I can take it outside with me (not that I need to, but if I did, I could). You can even drink on the metro (I've seen people drinking entire bottles of wine en route to their destination for the evening).
Also, on a similar note, I've taken to walking pretty frequently, to ration metro tickets and to make myself feel better about whatever love affair with French food I had that day. My sweet tooth has gone crazy in Paris.