On his Adobe blog, Mike Potter compares three technologies for communicating with their Flex product - JSON, XML, or AMFPHP.
I've created a small performance study that shows time to load three different types of data from a PHP backend... Data transfered as PHP objects from AMFPHP, data transfered as XML and data transfered in JSON notation.
The results of his test showed that, up to a certain point, the XML and AMFPHP were neck and neck (with JSON lagging behind). Once they reached a certain point, though, XML powered ahead, blowing past the AMFPHP library. Of course, just the sheer speed isn't all there is to an application, he reminds us - usability, serializing data, connection type, etc all need to be considered as well.