The EG-S is great and I leave it in my 5DMkII all the time. Viewfinder brightness is comparable to that of the 7D and I don't ever find the viewfinder to be too dark. IMHO these FF camera's should come with the EG-S as standard.
How does the EG-S screen work exactly? I'm only familiar with the split prism manual focus screen.
Is the EG-S difficult to install?
The normal EG-A screen has micro lenses that are aimed at the center of the frame to produce a bright viewfinder (but also too much DOF), the precision matte is more like ground glass and much more diffuse. I can't quite explain how it works optically, but when you look through the screen it is much easier to see what is in focus and what isn't. The EG-S effectively shows a shallower DOF.
This site ( http://www.focusingscreen.com/work/5d2en.htm ) shows how to change focus screens, I would recommend a Canon EG-S over a third party one as a third party screen may interfere with the metering accuracy of your camera (notice that you need to change a setting in the camera to account for the different screen in use).