There is an interesting synopsis of a Forrester paper over at Computerworld which seems to indicate that the sky is falling if you are building AJAX apps. While I agree that performance problems and lack of rich interactivity still put AJAX behind Flex or Flash for applications, I think the authors took it a little too far. I don't intend to go pay for the actual paper from Forrester, but I have to imagine that their conclusion is a little less alarmist.
Having built many AJAX and Flex applications I personally feel that it is harder to deliver as rich a user experience using AJAX as with Flex. My gripe with the article however, is that the synopsis specifically talks about power users as being disappointed. The reality is that only a small percentage of users of most applications are ever power users. Similarly, only a small percentage are novice users for very long. The majority of users are intermediate users. So be careful about choosing a technology based on the wrong population.