I have spent a lot of time in the last year studying and using AJAX technologies: Dojo (when I want a rich set of components), Prototype (when I want simple, lean AJAX support for forms), OpenLaszlo, some time spent with GWT, less time spent with Flex 2. For web applications that never have to be used "off line", these technologies meet my needs.
The Apollo developer pre-release looks compelling for applications that need to continue running when the user does not have an Internet connection. That said, I think that Adobe will get real competition from the open source OpenLaszlo system. Reason? Because OpenLaszlo is open source, great funding, and lots of Fortune 100 customers using it. I would like to see Adobe compete by releasing the Flex 2 SDK as open source (it is now free as in beer).