In older posts (or rather comments) I have mentioned that one of the biggest turn-offs for me regarding Flex was the fact that it was completely proprietary and closed… water-tight!
It seems that I wasn’t the only one with a problem so I just found out that Adobe plans to release source code for Flex as open source!
I don’t know how well this will work but it looks promising and as Yakov Fain mentions on his blog “there is already a decent number of developers who are interested in Flex and Adobe hopes that open sourcing Flex will bring more developers on board“
Dio Synodinos 
We are using flex for the UI in some of our corporate apps. I love it because it make building a sexy UI very easy and painless. Also since Actionscript 3 is really just ECMA script (i.e. close enough to javascript) then it is very fun to code.