I tried it at McDonald's recently in Manhattan and, like the other customers, was excited to use it.
I tried Raspberry Coke Zero (for those who were wondering).
It's very easy to use, and boasts over 100 flavors.
My only criticism is that my concoction was a little sweet. And my buddy who was with me also thought his Fanta Zero was also a little too sweet. A bit too much concentrated syrup, perhaps? It was still tasty though.