Why not Flex?
dimarco: We're using Flex + ZendAMF(PHP) and development is enjoyable. Serializing PHP data types that are also native to Actionscript remove the need to parse JSON and XML. On the wire the data is binary, thus small(no XML tag tax) and fast.Flex's default widgets and classes are all easy to extend and customize. You never need patches of js to customize some component on the bottom of the page.Adding features like drag and drop, navigation trees and tabs are almost too easy. Dividing your app up into loadable(and cacheable) modules can keep your application fairly lightweight, althrough still much bigger than a standard AJAX app.We(developers) will always be able to feel it when we're using a Flash app(before we even right click), but the CSS/Style/Skin support is good enough to make the app look inviting.