It is probably one of those things where Flex can be used in right ways and
wrong ways based on the experience (or inexperience) of the developer
putting the pieces together.
It will be interesting to see how it transpires. I do believe Flex stands
more of a chance than Silverlight and JavaFX. Silverlight will forever be
plagued by primarily only running on the latest browsers on Windows OSes.
When I ran the JavaFX samples about a year ago it was even more slow than
Flex (by quite a bit) so I am not sure if I hold a lot of hope on that front
either - especially since it's existence might be somewhat up in their air
should Oracle choose to not continue it's pursuit.