Your page will be more pleasant to use, when you can update parts of it without a refresh, which causes the browser to flicker and the statusbar to run.
Because you only load the data you need to update the page, instead of refreshing the entire page, you save bandwidth.
Disadvantages
Because the updates are done by JavaScript on the client, the state will not register in the browsers history, making it impossible to use the Back and Forward buttons to navigate between various states of the page.
For the same reason, a specific state can’t be bookmarked by the user.
Data loaded through AJAX won’t be indexed by any of the major search engines.
People using browsers without JavaScript support, or with JavaScript disabled, will not be able to use the functionality that you provide through AJAX.