Why Ajax Sucks (Most of the Time) "For new or inexperienced Web designers, I stand by my original recommendation. Ajax: Just Say No."
"Ajax breaks the unified model of the Web and introduce a new way of looking at data that has not been well integrated into the other aspects of the Web. With ajax, the user's view of information on the screen is now determined by a sequence of navigation actions rather than a single navigation action.
Navigation does not work with ajax since the unit of navigation is different from the unit of view. If users create a bookmark in their browser they may not get the same view back when they follow the bookmark at a later date since the bookmark doesn't include a representation of the state of the content on the page.
Even worse, URLs stop working: the addressing information shown at the top of the browser no longer constitutes a complete specification of the information shown in the window."