I do have Complete for CoP but I haven't messed around with it yet. I have a lot of experience with the Complete mod for SoC though and I don't play without it (at least until Lost Alpha finally comes out (which it never will)).
In the past I've used...
Green Zone - Makes the game less brown and more green, which makes it looks much nicer as a result.
Absolute Nature - Greatly improves on the original textures; especially if you're still using Windows 32-bit like me and can't set the texture resolution to the highest setting.
AtmosFear - Improves the weather system in the game.
Better Flashlight ++ - A better flashlight.
CoP High Quality Maps - Again, if you don't have a 64-bit system, this mod will give you the same quality PDA maps.
Real Gun Names - Changes the gun names to their real-life counterparts.
Correct Ironsights Alignment - This mod correctly aligns the iron sights in the game (they're a bit off by default).
FOV Switcher 1.7 - Allows you to customize the FOV (which is narrow as hell by default).
I'm sure some of these are made redundant if you're using the Complete mod which I plan on trying out shortly.