I'm not impressed by the y2k3 mod. It's advantage is that it is a total conversion mod that gives you a "new game". But...
* Models and units are taken from so many different sources they don't really match in colour, quality, abilities, etc. I want my missions to look good in game, and with y2k3 they don't.
* Anims are changed, but BIS original ones look more realistic. They way your soldier is holding his gun in y2k3 brings memories of "Doom" and "Castle Wolfenstein" to mind.
* It's not very stable. I've had several crashes with this mod.
* Different addonmakers use different techniques to give their addons special abilities. The word "confusion" is the first one that leaps to my mind when I am playing in this mod. Some units have extra features, some units has a lot of extra features and a lot of units have no extra features. This creates a feeling of not knowing what my playable unit can and cannot do during a mission.
* For the same reason as above, it is difficult to script and build missions with y2k3. Readme files are availiable, but they are not comprehensive or easy to understand.
I'm a big fan of total conversion mods, but there are much better options than y2k3 IMO. CSLA2 and FDF are my favourites. These mods have a uniform look, everything within them matches.