What if Jakob Nielsen had a blog?

Good AJAX: An Example

A good example of using AJAX to create effective page-area updates is found on E-Trade's stock quote pages. Each page shows a chart of a stock's price over time. Users can change the time-interval's length (say, from three months to one year) by clicking simple links. Appropriately, such clicks update the chart, but not the rest of the page, which contains other information about the stock. This partial updating works because it appears right next to the click zone and is a clear consequence of the user's action.

http://www.useit.com/alertbox/ajax.html

Web 2.0 Can Be Dangerous

As illustrated in a sidebar, an AJAX feature can work well on a website. And our testing did find one usable AJAX shopping cart. As always, the real question is not technology, but usability. If you use technology right, it can help sales. Still, the risk is typically too high with new technology because best practices haven't jelled yet. You can't just emulate designs you see around the Web — they're likely to be bad because they were hacked together by geeks drunk on the newest and coolest tech. And, sadly, "newest and coolest" usually translates into "untried and unusable" — and thus money-losing.

http://www.useit.com/alertbox/web-2.html

[Jakob forgot to mention "Ajax Sucks".]