I've begun using Flex for a production application, and I'm
quite impressed with the maturity of its design and development
environment. It has passed the first hurdles necessary to be used
for product-quality user-interface development:
Have cross-platform support
Be able to be compiled and deployed in an automated way,
without using the IDE
Have support for unit testing of code modules
Have support for automated product testing (just skated by on
that one - QTP integration appeared a few days ago)
The next level, though, is also important. The environment
still lacks any coverage testing (like EMMA for Java, for
instance). Flex also, as far as I can tell, requires an interactive
display environment to run in such as a browser or Flash player.
Automated regression test environments typically don't have
monitors and drive any UI behavior in a "headless" mode. Perhaps
Apollo will address this shortcoming?
Awaiting the next developments eagerly...