I have started learning Ajax from "Ajax in Action"-by Mannings. What I have realised is, that Ajax is pretty good considering user-interactive issues (saves you from lots of extra clicks & refreshes). But as most of the sites are turning highly graphical(atleast mine is), I am finding it a little redundant implementing Ajax. Though I fully agree with the argument that "
you should use it only if you require it"(disappopinted not being able to implement such a beautiful technology).