For years there has seemed to be a stigma in the development community about developing with AS (ActionScript). With the release of AS 2 there was a huge shift and many more developers made a transition to Flash; and with the ability to create classes, existing Flash developers were finally able to begin writing object-oriented code. AS has really matured over the years, and version 3 has raised the bar for object-oriented approaches by providing the ability to create packages, forcing strict typing through the compiler, and much more. In addition, the Flex environment has made it much easier to debug code and see errors or warnings while writing code.
From Flash to Flex: Moving to ActionScript 3
Peachpit, Written by Kris Hadlock