To AJAX or NOT to AJAX?

I had today a discussion on when to use AJAX and when not.

The idea is simple: the plain old request-response is usually a good thing, if you keep your pages small, which you should (CSS and JavaScripts should be external, thus a nicely built pure HTML page should be around 10-20K, not more).

The plain old request-response keeps your url meaningful - the state of the page is reflected by the url, which is a very good practice to follow.

The plain old request-response keeps your Back button function as it should, without any unnecessary tricks.

The plain old request-response keeps all your content available for search engines, which is usually what you want.

The plain old request-response keeps your user using what he is used to. No surprises.

So - when should AJAX be used?

• Auto-completion
• For fetching partial long lists based on some initial input
• Quick response to something insignificant, like showing the current results of a poll right after your vote is done, blocking you from re-voting (not that you cannot delete your cookie or session identifier and go back to the page to vote again, usually the vote is not registered per IP address)

Some additional insights on this topic can be found here:
http://webdesign.about.com/od/ajax/a/aa092506.htm