While the request is going to the server, the user can continue using the page for reading or selecting things, so delays are no more a feature of browsing.
Sites created using AJAX offer also the possibility to auto-fill input fields based on predictive text or previous user entries.
AJAX is the future of mobile browsing as you can create rich mobile applications for browsers making a richer experience to the user.
The latest version of Nokia Browser supports AJAX development.
More information on http://en.wikipedia.org/wiki/AJAX