Flash Actionscript Editor: Flex Builder vs. FlashDevelop
For doing complex development with Flash, it can help to have a more powerful Actionscript editor than the built in Flash CS3 editor. I've been testing Flex Builder and FlashDevelop, both of which can export both Flash or Flex (Flex is Flash with a library of components built for making web applications). Both seem to have their pluses and minuses. FlashDevelop is free.
As Nyquist says, Flex Builder can be better if you're doing more Flex (MXML) based development. In addition to having class view of the MXML and being good about hinting for MXML attributes, it's got a design view which FlashDevelop doesn't have. But, I've found a free AIR app that can mimic some of the functionality in the Flex Builder design view, called DesignView. It's not as full featured as Flex Builder's design view, but it can help to fill in the gap if you don't have Flex Builder.
As Sacks points out, FlashDevelop has very good code hinting, arguably better than Flex Builder and can also do error reporting right when you save a class. Flash CS3 can still be handy when you're creating vector graphics or components to use in Flash or Flex.
Additionally, AdvancedDataGrid is not included in the open source Flex 3 SDKs (which FlashDevelop uses); so if you want that, you have to buy Flex Builder. FlashDevelop also doesn't yet have any external source control integration, but FlexBuilder possesses the existing, stable source control options for Eclipse, which includes Subclipse for SVN.
For now, I think I'll keep and use all three for different purposes. Flex Builder for MXML development, FlashDevelop for Actionscript coding, and Flash CS3 for creating assets for use in Flash. What is your prefered Flash development tool? Leave a comment below.