Flash 5 is a bit different from the previous versions. The UI has a lot of floating menus and the toolbar changed a little bit.
In my opinion, the main difference is the incredible new actions and functions. The actionscripting is A LOT like javascript.
I would really recommend the program if you are a professional web designer and are very familiar with actionscripting, or simply if you want to learn the new scripting. You can have Flash 4 and 5 installed in your computer if you need to (not the upgrade)
So get in the list if you are into Flash because the new version is awesome. Let's see if they release it on time or the idiots from Adobe are going to get away with the lawsuit.-->
For all of you that are not aware of this, Adobe just sued Macromedia because supposedly Flash 5 has the same layout as some of the Adobe programs. *%$&%{ Adobe.
Now they are requesting to the court to hold the release of Flash 5. They are probably not going to win the case but this delay would really hurt Macromedia (and help Adobe and their stupid programs).
Now you know.