I briefly flirted with Flex and it reminded me of the era when I waited on a corporate custodian to release features that I needed yesterday. Flex might be open source but the culture is really hard to stomach; Adobe runs the show every step of the way, and nothing gets done without their blessing.

JavaScript improves at the pace of your users; i.e. you can roll out next-generation features by edict :-) it's free, portable, well-established, pleasant to work, and it's ours.