AJAX is a only great technique when used in moderation. It belongs in forms and page widgets, but should never be used for navigational purposes.
Call me a traditionalist, but webpages are pages, not desktop applications. Changing the way a UI component works on your website is unnecessary and confusing for your users.
Normal links shouldn’t change page content without warning and pages shouldn’t extend when you reach the bottom.
Use AJAX sparingly if you want to make a better user experience.