I never really got into the Counter-Strike skinning and model-swapping thing, mainly because I never really played any version of Counter-Strike seriously. However, with Left 4 Dead, I have all sorts of reasons to make my stuff sound more like I want it. Personally I want to swap out a couple of weapons but first I have to wait for the SDK to come out so I can properly import weapon meshes and animations.
Anyway I decided that I didn't really care for the original minigun sound, so I spent some time yesterday changing the sound from this:
The actual goods can be found at http://www.fpsbanana.com/sounds/6947
Here's a real one for comparison:
Some people I talk to like it, others hate it, but this is the reason content modding exists in the first place.