I think MX is a great program and still does most of the things anyone wants to do with flash. The major difference between flash 8 (the last) and CS3 is the addition of ActionScript 3.0. Everything in flash is run with AS. With the release of AS 3.0, from a practical standpoint, not much changed. There is difference in the way some applications are written in flash but it doesn't effect the practical applications much at all. For now AS2 does almost everything AS3 will.
That being said, I would wait until the next release of Flash is released to upgrade. Since AS3 is still relatively new with flash (as is Adobe), things are still being worked out. I'm sure with the next release there will be applications and all kinds of brand new bells and whistles which will be great, the only drawback is you won't be able to use them without AS3 (and purchasing the newest version, of course).
So, my advice for anyone using MX2004 or Flash 8 and thinking about upgrading would be to wait for the next version, CS4 or whatever they call it, and you will be much happier with the purchase. This will ensure that Adobe is committed to the application and locked into it for at least a reasonable period of time.
Keep in mind that this advice is from someone that purchases the newest application of Flash exactly 1.4 milliseconds after it is released.
Some guys just have to have their toys.
Hope that helps.