Once you code a bit in Flex, coding in the Flash IDE is very tedious. Of course, Flex has no graphical editor, that's when you find out ways to link with Flash. So the best combination I found so far is Flex + Flash.
Subclipse turns Flex's file browser svn capable, which is awesome (Flex 4 comes with it already). Beyond that, code snippet pluggins is all I would add to it but I haven't found one that suits me so far. I don't use it that way, but there are 3 articles on "Pimp my Eclipse" in Lee Brimelow's website.
