We have been doing a lot of work, on our product, with dynamic updates of web pages - you know the AJAX stuff. Most of the server side results are returned as XML and are delivered from different Web Services. Since I really like optimizations and like good and nice looking code I have been going back and forth in decision of how to return data to the client; XML or JSON. JSON are in some cases more effective but when the object model it tries to represent gets more complex you neither get nice looking code nor saves that much bytes.
Don Box led me to an interesting article which really shows this. This is exactly why I am using XML instead of the JSON notation.