Or so it seems from this article!
Makes sense if you think of it. Only part of the page is updated, so less bandwidth is used. And if you compress that data as well, you save even more.
Maybe AJAX is not just a novelty.
UPDATE [Feb 17 2006]: Some great additional articles are showing up in the comments. The catalogue demonstration is very cool.
UPDATE [April 29 2022]: AJAX, in the form of Single Page Application (SPA) Frameworks won.