I use FlashDevelop (http://www.flashdevelop.org/wikidocs/index.php? title=Main_Page). Flex Builder offers a lot, but it is $250 for a programming environment that should have been included in Flash in the first place.
NEVER develop anything in the Flash IDE. You don't start out understanding that everything is a class, nor how to get at the heart of things. The tabbing is terrible, the selection (using keystrokes) is non-standard, I haven't found a way to establish code hinting, and there are no macros/script inserts that I know of. It is not a programmer's tool, so don't waste time trying to stretch it into one. FlashDevelop has good code hinting, everything works in an intuitive way, and, with "snippets", you can develop more quickly. It is under active development, and problems are addressed very quickly; and there is also a large user-based support group if you run into trouble. And it is FREE! The whole idea is that you should not make any investment into any product until you determine whether you are going to use it enough to justify the costs.
