I’ve been using Flash since version 4 and think that I know most of the intricacies and workarounds in the application. I also consider myself an expert with XML and ActionScript, but when I tried to make the transition to Flex for the first time I was completely lost. The good news is that it’s really quite simple once you understand it. If you’re a developer, it’s even easier than using Flash because it provides accurate error checking and much more coding power.
From Flash to Flex: Integrating ActionScript with MXML
Peachpit, Written by Kris Hadlock