Please correct me if I'm wrong. From what I can read up so far, the Eg-S is pretty good replacement but the trade off is it's a little darker than the standard screen. There are alternatives such as the Brightscreen which retains the same amount of light similar to the standard but at a much more significant cost.
The EG-S is considerably easier to manually focus with. As a loose example, with the stock screen i found myself rotating the lens back and forth within say 15 degrees, searching for the exact focus distance. With the EG-S it's a fraction of that, probably 5 degrees or less, since it's so obvious what's in focus and what isn't.
That's the good news. The bad news is, as everyone says, it's darker, quite a bit darker. With fast lenses it's ok, but with say anything slower than f2.8 it's a little on the dim side. Workable, but not bright.
You know when you put an old Olympus OM camera up to your eye and go 'Wow!' ? Well, it's not like that.
You know when you first got/tried a 5D after shooting a 20d/40d/etc., and go 'Wow!' ? well, this focusing screen basically un-wow's that kind of comparison.
That said, my EG-S is still in action because of the keeper rate. Out of 30 ish lenses, only 3 of mine are AF, so that's why
