Café Marly is in a great location -- right next to the Louvre, Arc de Triomphe de Carrousel and Tuileries. The perfect location to people-watch on the balcony and enjoy a decent meal.
From my experience, service was good. However, I find they tend to cram everyone together at one end of the balcony so there is not much space between tables and customers. I also was a little less than impressed with the fact that the chair covers were dirty and moldy and had hair all over them. Prices were relatively expensive, but that can be expected for Paris in general...especially if you are eating at a location so central as the Café Marly.
Would I go for the food? No. For the experience? Yes.