Freeman,
HI and welcome to SimV.
That is hard to answer because as they say "to each his or her own".

Which means that you'll likely get a lot of different answers to that question. In the end... you'll have to do your homework and decide for yourself.
Have to say that I think it is a bargain for what you are getting even at the full retail price....and that price has started dropping a lot.
The biggest question to ask yourself before you consider "is it worth it" is if your hardware will measure up to all the "promise" on the box. FS2004 is amazing when compared to fs2002....... but having all that promised complex stuff comes at a price. If you have to turn down the sliders on many of the new systems and effects... then you will be getting less than what you THINK you are getting.
That being said..... there are many improvements over fs2002.....too many to fully mention. One that seems to work regardless of the hardware setup you have is the improved ATC. It works FAR better than 2002. That alone might be worth the upgrade ;). Now vectors seem to make sense most of the time. The communications menu for ATC is improved and has more possibilities.
The visuals are stunning..... particularly the clouds and weather. This upgrade is a LOT about visuals. Stuff just looks more "real". There is more eye candy. Screen shots can almost look like photos. But those wonderful visuals demand a fast machine with a really good video card to fully realize them without significant frame-rate hits. "Good" is pretty much defined there as one of the top one or two cards.
The AI traffic system works much better now. Planes fly to places they didn't before and they seem to "behave" better. (But they still sometimes seem to be flying drunk ;) .)
From my point of view, the aircraft list they supplied this time is not all that great. But the number of add-ons available for fs2004 is immense. Ditto for scenery.... and the add on scenery is better.
BUT.......... there are few here who seem to have everything .... and I mean
everything..... set to the maximums and acheive consistent frame rates of at least 20 FPS on short final into a large international airport with lots of AI traffic around. Most of those folks seem to have cutting edge gamer machines. Keep that in mind. If your machine is "less" than that.... you will likely have to turn some stuff down to make it
both as good as you can get it visually and
also have frame rates that let you fly important manuvers effectively.
Go back and scan all the thread headers in the fs2002 forum back to the time of release...and pick the ones that seem to interest you in your research to read. That will give you a "real world" picture of what others have experienced with the new sim as we have all "figured it out".
This sim is driving me nuts.... but I still think it is worth it. ;D
best,
..................john