AJAX is being used in more and more places, often when it doesn’t need to be…and when it shouldn’t. With all the talk about how you can use AJAX for everything, the real question is when should you.
You should use AJAX in places where it will improve the user experience. One example is with polls. Why should a full pageload be required just to vote in a poll (or view the results)? That’s a waste of your users’ time, and a waste of your server resources.
Basically, you want to use AJAX for things where an extra pageload would be irritate the heck out of everyone. Suppose you have a star-rating system, like on Netflix. Wouldn’t it be horrible if you had to sit through a page refresh every time you rated a movie? Use your own judgment.