At JavaLobby Marc Domenig wrote a really good article entitled Rich Internet Applications and AJAX - Selecting the best product about choosing the right approach to building AJAX enabled applications on the Internet.

He Writes:

There are hundreds of criteria for evaluating RIA and AJAX products. So many that it's easy to lose focus and misjudge priorities. This essay proposes a decision tree that leads through an evaluation process. It asks for the most distinctive requirements and product features in a top-down sequence, discussing the essential differences between technology options.
I think the article hits some key concerns. There are a lot of things to think about when changing an existing application or building out a new one that uses AJAX. I happy to see someone out there is thinking about these issues and helping us tackle them.