Since it's emergence as a mainstream technology, AJAX has proven it's usefulness. AJAX technologies are now employed by companies such as Microsoft and Google, and are used by millions of web pages.
AJAX does, however have drawbacks which take away from it's shine. One of the main concerns with AJAX technologies is with it's impact on usability - certain screen readers and other accessibility devices might not respond well to dynamically updated web pages.
Important issues emerge when a fast internet connection is not available. This can lead to websites being very difficult to use, because AJAX is often used for functionality which requires responsiveness - such as responding to the keys the user types in a search box, by updating a list of matching items. On a slow internet connection, this might lead to serious usability issues.
Another problem can arise due to the fact that AJAX relies on JavaScript. But many users don't like JavaScript, and all modern browsers provide some mechanism of preventing JavaScript code from executing. If the user has JavaScript disabled, the website will not be dynamically updated. Although improbable, it is also possible that some users visit the web page with browsers which do not support JavaScript - in which case, again, dynamic content will not be updated.
On the server side, employing AJAX technologies can lead to very high number of requests to serve, which might be difficult to cope for the servers. But companies usually take this into account, and purchase hardware which can cope with the increased load; this is rarely an issue.
