I have been a Flash Develop devotee for that last 3 years. Even after I bought my Mac I made sure to install and keep Parallels updated as to not lose the feature-set I love so - Flex asset compile-time embedding, custom class and package recognition, a unified project work space, and the simplicity lacking in some of the Eclipse-Focused IDE options. I have always found that development environment inside the Flash IDE to be a little lacking, but I really like the design tools the IDE offers a well-rounded game developer.
I have been using both together for a number of years. I never bought a copy of CS4, so I had been using the Flash Develop / CS3 combination for quite some time. In CS3 I would create assets and save them out as SWF files and in Flash Develop I would create my AS3 Flex project and string the whole thing together by loading (at run-time) or embedding assets at compile-time.
This was a great way of working until last week when I updated Parallels and for some reason Flash Develop stopped being able to work with my Mac folders. This forced me to create separate Flash Develop project folders on my Parallels Windows partition and copy the files back to the Mac to be able to work on them in the Flash IDE.
Luckily I purchased a shiny new copy of Flash CS5 last week and decided to see if I could replicate the basic features of Flash Develop in the IDE. First I found the almost hidden "Project" Panel and created a new AS3 project. I created a .fla in the root and a simple package structure for my new project starting there. Flash CS5 figured out the structure right away and even created new class files with the right package name! Score!
Next I switched the IDE to the "developer" view which got rid of the timeline and replaced it with the Project Panel, Debug window and large editor screen. My final test was to see if I could create a custom class in the new package structure and have the IDE recognize it. I placed a new class file into the package, and then in my Main.as I created an instance of it. Without having to include the package or the class, the IDE recognized it and I was able to see the constructor (but not it's parameters). I could see the public properties and methods as well as the parameters to them though (including the constructor) after I created an instance of the class.
This was enough to get me encouraged and moving in the right direction. I will continue to play with it, but for now it gives me most of what I could want in a single Flash (not Flex) development environment. I will still be using my trusty Flash Develop environment for pure Flex or AS3 projects that don't require heavy IDE involvement.