I'm a long standing user of flash, having used the program for about 7 years. This update is one of the most significant yet, inline with the upgrade that version 5 presented.
3D is a massive edition, the edition of 3D content in Photoshop CS3 Extended I would say was a big success as it provided tools for texturing 3D models etc. Adobe have now incorporated similar technology into flash that allows us to create 3D animations based on 3D content, or existing 2D content that you may have.
In terms of interface, the program has a complete overall, inline with what Adobe are doing with the rest of the CS4 Suite, the timeline is now at the bottom of the screen like you might be used to if you are a Director user, however you can easily move it back to the top if that suits you best.
Animation has been just ripped open in this version and built from the ground-up. There are timelines, sub-timelines, no-timelines at all, its too hard to explain here, you might need to look at some tutorials, but basically animation has been completely redefined and we now have a huge array of options for creating animation.
The bone tool is a great tool if you are into cartoons and character animation and the like but I don't know how much I'd use it outside of this limited scope.
Overall, this version of flash is a massive upgrade and really the biggest upgrade of the entire CS4 suite, with a huge array of new features. Is it worth upgrading? I think that depends on your workflow and what you use flash for, but it certainly opens the way for a new audience to start using flash with its extended features.