There is still a very important lack in Intellij that is still blocking me from switching:
In FDT and Flash Builder you can link a global library folder containing ".as" clasees that can be easily shared in all the projects. In IDEA this is more restricted: the Project/Modules structure that should be very similar to the Workspace/project structure of FDT/FB, it's not exactly the same.
Let me explain: I've always used (both in Flash bulder and in FDT) a global _CLASSES/AS3 folder that i keep on C: (or Users/myName in OSX) attacked as a linked library in all my projects: inside it I keep all the packages that i use (com, org, etc...): it contains ".as" files and not swc, I prefer this because when i change some classes i do not have to do the whole swc again (HUGE).
In intellij, Inside a project structure, you CANNOT use the SAME SOURCE FOLDER in more than one module, so you are obliged to link you _CLASSES folder in the dependencies tab. This is ok for code hinting but then, when you compile, you have a "package not found" error. If you link your _CLASSES folder as a source folder (using ADD CONTENT ROOT) everything goes right but you can't use the same folder as content root in another module until you unlink it from the previous one.
I've managed the whole thing adding "-source-path C:/_CLASSES/AS3/" as a compiler option in the flex compiler settings: doing this the compiler includes my global folder when compiling. I think it is boring.
in FDT (Same in FB) I've created a kind of project template that I use now for every project I start, already with all the folder structure and linked classes libraries that I always use. I've reproduced the same in Intellij but to make it work I have to copy the module and rename the .iml file (It does not rename it automatically), the import it as a new module in the project. Boring. I definitely love the customizable "Project Template" feature of FDT.
Said that, i must add that Intellij X is A-W-E-S-O-M-E!