Flex -- Flash's server-side approach -- has enjoyed success in recent years with the heavier demands being put on GUIs enabled on the web or Rich Internet applications(RIAs).
This however hasn't stopped Sun Microsystems from spinning its own RIA alternative dubbed JavaFX.
Chet Haase who has worked on the development of both platforms provides a post comparing both technologies.
Read Chet Haase's post 'JavaFX and Flex how do we compare?'
Read Chet Hasse's blog