The new Flash CS3 application is excellent. The new menu and pallette system they have build in is very quick and non-intrusive. The general UI is very responsive in Flash and Dreamweaver. In version 8 and other big IDEs out there things have been slowing down, but this release by Adobe things have really sped up. However if you have been developing Papervision3D and using flash 9 alpha, FlashDevelop etc you may have a bit of setup to do to use Papervision3D or some of your old kits that are having trouble compiling.
Compiling AS3 with Flash CS3 is pretty simple the default classpath to the ‘flash’ namespace is already set by the application to $(AppConfig)/ActionScript 3.0/Classes. You can also add your class library path to where papervision3d is located in Edit > Preferences > Actionscript > ActionScript 3.0 Settings… (button). I keep mine in [drive]\Projects\FLASH\classpath\as3\.
FlashDevelop/Flash9 Alpha AS3 Users
One problem is that if you have been using the alpha flash 9 and used FlashDevelop or another IDE you might have some classes to remove from your class path before you can jump right in with Flash CS3. These would be anything in the ‘flash’ folder in your classpath and probably some type classes in the root such as Object.as (intrinsic classes you may have added to complile to AS3 or code completion like in FlashDevelop).
The best solution if you do not want to clean that manually is setup a new classpath folder and then move new AS3 libraries in there. Or, you could setup a new folder and keep the existing classpath until your projects are swtiched over. I keep a classpath for all versions I currently support in my development.
These paths can be added to your Flash IDE, your external flash actionscript IDE like FlashDevelop or other compilers that may come along now that Flex SDK is opensource.