As far as CS4 and programming is concerned, the best feature is a new object called Vector. It's an Array but it only allows for one type of object (for example, Vector.<uint> or Vector.<String>). We found it runs at minimum twice as fast for computations than an Array.
3d stuff is actually pretty nice. It's nice to actually be able to do a perspective tilt (think Star Wars intro).
Best feature in CS4 though is some new features in the interface, most importantly the Library search feature that means you can stop hunting through multiple folders and such for your symbols. Also, multiple select in the library means I can stop clicking all 100 linked movieclips to "export in first frame" and do it just once now.
Overall, it's probably worth updating... maybe. I think for many it'll be worth the $200 but for others it'll be not up their alley. We mostly needed the Flash 10 for the Vector object and a few other things. But of course, if you are Flexy you can do that already... I think.