Baekdal.com has a good article on Usable XMLHttpRequest in Practice:
One of the great benefits of XMLHttpRequest is that you can use it to make complex WYSIWYG. This has previously been hard to do on the web. Take a “build your car” feature that you often find on sites of car manufacturers. This could be greatly enhanced by the use of XMLHttpRequest. When the buyer selects something, you can connect to the database, recalculate prices, change the image or change other options – without reloading the page, on the fly.
Full article here.