Over at Schematic, there is a wonderful summarization how Forrester missed the mark when comparing AJAX and Flex. I won't bother summarizing it here. You should read it for yourself.
As a designer/developer having built large scale Enterprise web applications using both AJAX and Flex, I have to stand up and whole-heartedly agree with Schematic. I guess the folks over at Forrester have never actually built an AJAX application and had to deal with browser compatibility issues, memory leaks and the obvious sub-optimal UI you end up with in the end.
While much of the Forrester report seems to be incorrect, I have to disagree most with the statement that AJAX is faster to develop and easier to learn. I have had the opportunity to build the same application as both an AJAX and a Flex application and I can say without a doubt that the Flex version took about 40% less time to develop, has a much better UI and works seamlessly on multiple browsers and platforms. When we made the decision to build it in Flex we had very little experience with Flex as most of our previous applications were all AJAX. Yet, it was still faster to develop in Flex.
In my opinion, if you are building or rebuilding a new application, as opposed to enhancing an existing one, Flex or Silverlight are the obvious choices. OpenLaszlo is out there too, though I have not seen a compelling reason to choose it over the now OpenSource Flex. Now, before someone comments with "OpenLaszlo can generate DHTML" let me say, that generating DHTML seems to be something like Un-alchemy where you turn Gold into Lead. While Silverlight looks promising, Flex is a few laps ahead in the RIA race.