Over the weekend I decided to start playing around in the .xml and creating my own mod. I was reluctant to do this earlier because I was both too busy playing, and also because I have messed with .xml 'moddable' games in the past that were an absolute nightmare to work with. I mean, stuff to modify a single weapon or unit could be spread out over 3 or 4 .xml files and if one of them doesn't jive with the others the mod is ineffective at best. The Empire At War games are--in my opinion--the worst for this kind of infraction.
So I started to mess around in the various files, I read the primer on GC2 modding and set about to throw a mod together. In a few hours I had a couple new weapons and some new armor, new invasion tactics and a couple new techs. I hit a bump when I totally messed up some .xml syntax that took awhile to clean up (the game kept crashing on startup), but once I got it right everything worked great.
Now my mod comprises a full new tech branch, several new weapons and defenses complete with graphical models and sounds for each (just recycled from what ships with the game), a couple of new improvments, etc. The whole experience was really great, and very fun.
I found the .xml data structure to be wonderfully well thought out and robust and easy to understand. If I ever move farther in programming I hope I can layout my work this well.
Now, if I can stop modding long enough to actually play...I think that's the next step