Properly implemented, AJAX can make site navigation much faster for the end-user. Having the browser refresh and render a page takes a lot of time. Preventing this, by requesting and updating only portions of the page, is very efficient. However, without a good implementation or proper infrastructure, AJAX can bring your server to a stand still with excess or unnecessary requests.
While it’s true that Flash was once the buzzword that AJAX now is in web development circles, the idea that AJAX will replace completely flash is unfounded. While entirely flash based designs are becoming increasingly uncommon, flash still serves a purpose for many multimedia and graphical interface elements.
While AJAX has given many great possibilities in web development, consider the technology carefully before implementing it on your own site. Don’t break the likability, accessibility or navigation (back/forward buttons) of a site just to use AJAX. Use it where it is the most efficient way to make something work.