If you've been following the blog shooting war between the boosters of XML and JSON and have been scratching your head as to which one to use, Dare “Carnage4Life” Obasanjo has written a great article with an excellent executive summary:
If you're too busy to read them, here's the executive summary. JSON is a better fit for Web services that power Web mashups and AJAX widgets due to the fact…that it is essentially serialized Javascript objects which makes it fit better client side scripting which is primarily done in Javascript. That's it. XML will never fit the bill as well for these scenarios without changes to the existing browser ecosystem which I doubt are forthcoming anytime soon.