I think AJAX looks really cool. I haven't had a chance to really dive in and get to use it, but I looking forward to it. Using some other people AJAX-intensive sites I can really see where it can be handy. But, there is one thing about some sites using it that drive me Crazy! It's the whole problem with the back button. In AJAX, since you are not really leaving the page, the back button does not work as expected. Hitting the back button does not load the previous content (as one would expect), but in fact sends you the previous page (the way the browser interprets this), therefore, taking you away from what you want to see.
Now, granted, maybe I am not the "target" user. Maybe I am a bit of a power user. I don't like using the mouse. The less I have to go to my mouse, the faster I can do things, and the less my right wrist hurts (blasted carpel tunnel). So for me, I love using things like the space bar, ALT-TAB, and especially the Back Space button to go back in the browser. And, even when I am using my mouse, it too has a "back space" button. Both back space methods are not AJAX friendly, but are highly incorporated in my daily life.
Gmail seems to work well with the back space button, and I know they are hugely into AJAX, so I know it can be done. Maybe some of these people should reprioritize the user experience over the page load time.
Seems silly to me that a methodology such as AJAX which is meant to create a better user experience is acutally slowing me down and making my browsing more complicated. I can understand catering to the general user, but not at the expense of a power user.