So far I have found it is very cool and helpful to run some simple graphics effects and XMLHTTPRequests for single form fields or small forms, but designing the “single page” web app isn’t quite suited for standard web site right now. I say that for on simple reason, useability. Yes, useability. People have it so implanted in the mind on how to operate a web site, when you stray from the standard practice (like back buttons, urls), people don’t like it. Sure, you can deal with the back button issue, but why work so hard to do that when it’s so fast to do things with a new page.
I just find that using AJAX to compliment a web site form works best for now. However, say you want to build a complete application running over the Internet, then doing things the AJAX way might make things interesting. But, you’d have to rid yourself of the toolbars and run the app in a clean window, because you will be forcing the useability into your app, not into a web browser. (even though it still runs in your browser)