A good example of how to not use Ajax.
Few months ago, Blizzard Entertainment announced that Starcraft 2 is on the way and they opened a
web site to make sales promotions. In the
sections where you see details about units and buildings in the new game, clicking on a unit will change 90% of the page with Ajax (no refresh in browser). The problem is that many heavy pictures and flash are loaded using this method and this makes the browser freezing for a while. I tried it with different browsers and different computers but it won't help. The browser always freezes.
Think twice when designing Ajax-enabled web page. Downloading too much data with Ajax can perform troubles. And troubled users won't come back to your web site.