If you can do all the rich interface stuff you want to with existing browser technologies then probably not. One of Flex's big benefits is it has a far lower learning curve than the AJAX framework route. If you are already past that, and you have existing AJAX applications I'm not sure I see a reason to switch.
As a part-time developer (and full time architect), I am not as entrenched in any JS framework technology, but for other projects in which I may be directly developing some RIA apps, how would you say Flex compares specifically with something like JQuery? What about integration with iPhone devices or cloud applications such as Google Apps or Force.com?
Paul Sturrock wrote:Apple have gone to extra lengths to prevent Flash (and other languages) running on the iPhone.
Now Adobe is coming up with Packager for iPhone and that will enable Adobe's Action Script to work on iPhone. It is going to hit markets soon.
...and Apple have changed their licencing to ban access to the iPhone API through any intermediary layer. Adobe is no longer actively developing their iPhone packager. Flash is not going to run on the iPhone any time soon.