When is it appropriate to use AJAX?

Question

When is it appropriate to use AJAX?
what are the pros and cons of using AJAX?

In response to my last question: some people seemed very adamant that I should only

Answer 1

I think the advantage of using ajax technologies isn't only for creating better user-experiences, the ability to make server calls for only specific data is a huge performance benefit.

Imagine having a huge bandwidth eater site (like stackoverflow), most of the navigation done by users is done through page reloads, and data that is continuously sent over HTTP.

Of course caching and other techniques help this bandwidth over-head problem, but personally I think that sending huge chunks of HTML everytime is really a waste.

Cons are SEO (which doesn't work with highly based ajax sites) and people that have JavaScript disabled.