There’s a good discussion going on over at Ajaxian about specific times when Ajax should or should not be used based on a posting by Alex Bosworth (10 Places You Must Use Ajax).
From Alex’s article:
For heavy use applications such as a webmail client or a blogreader, users have the luxury of time to learn new UI concepts, and the frustration of interacting with a slow interface. This kind of application is a perfect opportunity to leverage Ajax everywhere. The more frequently users use an application, the more Ajax should be powering that use.
A good reminder for sure, that ultimately it is not the technology that should dictate developer choices, rather it should be whatever will best serve the customer (the users!)