http://www.kottke.org/05/05/ajax-weblogs

Check out the post and new drop down box at the top. I have to say (again) that I don't really think this is that great. What's it saving me? A quick page refresh? Sure there's not quick refresh but there's also no indication that my command has been received. Here he's switching out 70% of his page. At that point I think you need a page refresh and not an AJAX call.

So far I like AJAX for things like instant feedback simple searching that keeps you from having to do it a bunch, I really like live updating potential answers in a text box like Google Suggest, because it's extremely transparent to the user and things still work if AJAX fails. I just have to say that I don't like it for replacing such a large portion of the page.