I've been doing lots of ajax as of late. At first my thought was: "been there, seen that!" as i already did lots of asynchronus stuff (using hidden iframes). But i'm totally sold now. Here's why:

1. While using iframes I had to manage my call backs from within the returning page (i.e. server output needed to know about the client side control structure, and needed to resend some post operations data other than a mere success or failure message). Now it's totally client side! Using callback functions (with their closures! you dont get to lose the request initiating environment!!) is so very sweet to work with!
2. Handling server misbehaviour!. You dont rely on the iframe to always load any more, you can easily detect when a request times out (without writing your special timing routines for that!)
3. All you communicate is data now. And I even dont use XML, JSON is my winning card here and it integrates oh so easy with the front end logic (our back end code is written in C, thanks for whoever wrote libjson, a life saver for us).